After comparing OD&D EHD to 5e CR for preliminary conclusions, and after having estimated EHD for 5e, we now have equivalent hit dice as a measure of strength for many monsters in both editions and are finally able to directly compare how much more or less powerful each monster became.
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Mapping the monsters
Not all the monsters map 1:1 by name. Where they do not, but there is a reasonabe equivalent, I use that. I ignore monsters where there is no such replacement (e.g., the various ants and giant beetles). In a few cases, OD&D monsters span a range of hit die while there is only one form in 5e, like the animals, elementals and sea monsters, roc. In such cases, I compare the 5e form against the strongest version. For dragons, OD&D EHD are given for medium sized ones, which I compare to adult dragons. Lastly, especially for man-types, there are a lot more in OD&D than there are comparable NPCs in 5e. There are for example bandits, brigands, pirates, buccaneers, all of which would map to bandits in 5e. Instead of repeating, I just select one of them for comparison. In total, 138 of 182 entries in the table are mapped and compared.
The table shows the OD&D name in column "Monster" and the 5e monster Name mapped to (if different) in column "Mapped to".
Comparing monster strength
We list for each monster their EHD in OD&D (column "EHD") and in 5e (column "5EHD"), and compare how much EHD they won or lost (in column "Diff") in absolute terms. This shows directly how much the monster weakened or strengthened in 5e compared to OD&D. If it is 0, there is no change in strength. If the value is positive, you need a higher level group to defeat the monster than before. The table is sorted by this to show the winners and losers.
We also rank the monsters for each edition by EHD, with the rank position (columns "R" below) indicating their relative power among all monsters in the given edition. We can see how much up or down in rank from most terrible to weakest monster they move between editions, as a result of both their own and all other monsters' changes in power (column "Shift" in the table).
The gold dragon, beholder, lich, titan, and black pudding lack EHD for OD&D. As they were among the most powerful, I put them at the top of the OD&D ranking, a bit overstating it for the pudding. The rust monster and ochre jelly also lack EHD, I put them at the bottom of the ranks.
With the exception of the iron golem, the strongest monsters in 5e are stronger than those in OD&D. They are intended to offer challenges all the way up to character level 20, while OD&D was intended for play up to lord level (around level ten).
Dragons, golems, sea monsters, lich and beholder are near the top in both editions. Elementals, basilisk, vampire, treant, black pudding and will-o-wisp from OD&D are displaced by demons, angels and giants at the top of the rankings in 5e.
The losers
The golems were brought in line, especially the terrible iron golem wo was at #1 with his insanely high outlier score. He still remains one of the powerful monsters. The stone golem also suffers a bit, the flesh golem less so.
The will-o-wisp drops most dramatically from deadly to nuisance. It was on par with a red dragon back in the day, now it can be defeated by a first level party. Maybe because it does not seem a will-o-wisp should be such a terrible thing?
The basilisk suffered the same fate as the will-o-wisp. Here however it is a theme to soften petrification, once a single save-or-die roll. The cockatrice drops from terror to inconvenience. Maybe to have some petrification challenge at low levels, but even the basilisk can now be faced by low level groups, so it would have made more sense to keep him for the high end (the middle is still covered by gorgon and medusa). I think this was meant to make these monsters more accessible, but it does not play well with the terror invoked when one sees petrified adventurers.
The elementals lose about half their power and drop from among the strongest creatures in the game to solid: no longer can you summon one of the games strongest monsters with a level five spell. The ability of the players to summon them was balanced by the fact that you could lose control of them, so it was a dangerous proposition. I'm not sure this was a great design, but the echos still persist uselessly to 5e's conjure elemental. The new elementals are more comparable to what used to be the smallest ones.
Among the undead, the vampire is the most prominent monster to be nerfed. While he remains strong and at a high rank overall, he loses nearly a third of his former power. This may be because his warrior and spellcaster variants are stronger than the normal vampire. The spectre, once the second most powerful undead in the original monster list, is mostly harmless in 5e. I suspect this is because with the ghost, you already have a high level incorporal undead that is very similar, and this allows you to have "ghosts" at lower levels of play.
The black pudding, once the terror of the dungeon, drops to a challenge for a beginner party. In his Greyhawk campaign memories, Gygax talks about how Robilar ran into a black pudding and lost his magical boots to it. I was puzzled, as Robilar must have been pretty high level by then (it was dungeon level six, he was fleeing from a pair of black dragons), while I knew the pudding only as a middling monster from 5e. A black pudding was one of the deadlier encounters you could have, even if we lack EHD for it.
If we considered the plesiosaurus or kiIler whale as "sea monsters", they would dive deep. The tree, giant crab, and giant weasel are on the table and do so too. Huge beasts in general tend to be weaker. I like the old version better, where huge animals remain a threat into high levels.
The ettin, once a chap for his giant brethren, now is the little man's giant solution.
Monsters with multi-attack routines or paralyzation have been dampened, the number of attacks much reduced. Among them the treant, roper, manticore, ghoul and carrion crawler. The first two used to be super tough.
Monsters with poison have been weakened, as it is no save-or-die effect any more. They include the phase spider, giant spider, giant scorpion, and giant poison snake. The phase spider has an inflated CR, likely because its poison could one-shot a wizard and its movement cannot be blocked. Any of those formerly could spell death even for high level adventurers. Not any more.
The middle ground
The titan and lich remain among the most terrible foes in the game. The titan however has a grossly overinflated CR for what he does. The purple worm, hydra, and ogre mage are also about where they used to be. The worm was one of the strongest monsters before the high end invasion from lich, dragons, beholder, balrog and golems. He still is one of the dangerous monsters in absolute terms.
I was surprised to see the medusa, gorgon, hydra, and wyvern not losing or even gaining ranks or power. All of them used to be quite deadly due multiattacks, petrification and poison, and in 5e all these attack forms were weakened, plus they are no high-end threats any more. I thought originally this could be a CR labeling issue, but it seems they are really not losing power -- maybe their other stats have been beefed up to make up for the loss of special attack effect, or the 5e simulator gets them wrong.
The invisible stalker is staying comparable, too. While the stalker had only half the HD of an elemental in OD&D, its long term servitude made it a more powerful spell. 5e has the stalker as a CR 6 elemental, and the spell to summon it is also conjure elemental, cast on spell level six. The CR is misleading, in absolute terms elementals are still stronger than the stalker.
The winners
Dragons. The games eponymous monsters have become much stonger and rule the chart in terms of absolute power. The ancient ones are not even shown here, they would be approximately 80 EHD. Even the adult dragons win massively.
The balrog is a large winner, and makes it into the top 10, outside of dragons, kraken, beholder and lich it is about the worst thing you can run into. When it comes to outsiders, the efreeti and djinni (not shown due to technical issues) also win, despite lacking their fairy-tale wish-granting powers.
The kraken as the biggest sea monster is even more terrible than the humongous sea monsters of old, followed a little down the list by the dragon turtle. I think these offer fair equivalents for "distract and flee" ship encounters. However, I believe the idea was these sea monsters were animal intelligence dinosaur, giant octopus and whale types, and those fail to impress (see above).
All the giants are stronger than they used to be, which makes the classic G-series adventures more of stealth and avoid missions, than direct assaults that would be quite deadly for the PCs. The troll may be a fluke as the simulator could not handle regeneration, it should be possible to deal with him at a lower party level using torches.
All the were-creatures appear stronger, I believe because they can be wounded only with silver or magical weapons, while nearly all other monsters only have damage resistance, not immunity.
The berserker used to be a slightly beefed up normal man, and appeared in introductory dungeon B1 as level one wandering monster. Now it is a threat with a lot of hit points and a tough fight for first level characters.
The wraith switched places with the specter in the pecking order of undead. What surprised me was that he still gains in effective power. I felt all the undead that have level drain were horrifying back then and are weaker post level drain, but that is for another investigation to explore.
| Monster | Mapped to | EHD | 5EHD | Diff | OD&D R | 5e R | Shift |
| Golem, Iron | | 98 | 28 | -70 | 6 | 14 | -8 |
| Will-O-Wisp | Will-O'-Wisp | 27 | 2 | -25 | 17 | 85 | -68 |
| Basilisk | | 25 | 3 | -22 | 18 | 71 | -53 |
| Earth Elemental, Large | Earth Elemental | 39 | 17 | -22 | 7 | 27 | -20 |
| Fire Elemental, Large | Fire Elemental | 32 | 12 | -20 | 14 | 44 | -30 |
| Water Elemental, Large | Water Elemental | 32 | 17 | -15 | 13 | 26 | -13 |
| Vampire | | 38 | 23 | -15 | 8 | 17 | -9 |
| Golem, Stone | | 37 | 22 | -15 | 9 | 18 | -9 |
| Treant | | 32 | 18 | -14 | 12 | 25 | -13 |
| Air Elemental, Large | Air Elemental | 22 | 13 | -9 | 21 | 36 | -15 |
| Phase Spider | | 10 | 2 | -8 | 45 | 84 | -39 |
| Tree | Awakened Tree | 11 | 3 | -8 | 42 | 63 | -21 |
| Spectre | Specter | 9 | 2 | -7 | 55 | 87 | -32 |
| Carrion Crawler | | 9 | 2 | -7 | 54 | 79 | -25 |
| Cockatrice | | 7 | 1/2 | -7 | 62 | 120 | -58 |
| Giant Spider | | 7 | 1 | -6 | 61 | 93 | -32 |
| Giant Scorpion | | 9 | 3 | -6 | 53 | 69 | -16 |
| Manticore | | 9 | 3 | -6 | 51 | 65 | -14 |
| Griffon | | 9 | 3 | -6 | 52 | 66 | -14 |
| Ettin | | 12 | 6 | -6 | 39 | 51 | -12 |
| Ghast | | 7 | 1 | -6 | 60 | 92 | -1 |
| Lord | Knight | 9 | 4 | -5 | 50 | 57 | -7 |
| Giant Snake, Poisonous | Giant Poisonous Snake | 5 | 1/2 | -5 | 79 | 115 | -36 |
| Dryad | | 5 | 1/2 | -5 | 78 | 110 | -32 |
| Djinni | | 7 | 3 | -4 | 59 | 76 | -54 |
| Harpy | | 5 | 1 | -4 | 77 | 97 | -20 |
| Mummy | | 7 | 3 | -4 | 58 | 64 | 11 |
| Sabre-Tooth Tiger | Saber-Toothed Tiger | 6 | 2 | -4 | 68 | 81 | 12 |
| Giant Snake, Constrictor | Giant Constrictor Snake | 6 | 2 | -4 | 67 | 77 | 15 |
| Giant Crab | | 4 | 1/3 | -4 | 87 | 121 | -34 |
| Giant Wasp | | 4 | 1 | -3 | 85 | 98 | -13 |
| Roper | | 15 | 12 | -3 | 31 | 42 | -11 |
| Mind Flayer | | 15 | 12 | -3 | 32 | 43 | -11 |
| Bear | Brown Bear | 5 | 2 | -3 | 76 | 83 | -7 |
| Lion | | 4 | 1 | -3 | 86 | 102 | 8 |
| Giant Octopus | | 4 | 1 | -3 | 84 | 91 | 18 |
| Giant Weasel | | 3 | 1/3 | -3 | 99 | 126 | -27 |
| Ghoul | | 3 | 1 | -2 | 98 | 106 | -8 |
| Golem, Flesh | | 17 | 15 | -2 | 27 | 32 | -5 |
| Mastodon | Mammoth | 17 | 15 | -2 | 28 | 33 | -5 |
| Medusa | | 14 | 12 | -2 | 34 | 39 | -5 |
| Shadow | | 3 | 1 | -2 | 95 | 95 | 0 |
| Gargoyle | | 5 | 3 | -2 | 74 | 67 | 7 |
| Myrmidon | Veteran | 6 | 4 | -2 | 65 | 55 | 10 |
| Minotaur | | 6 | 4 | -2 | 66 | 56 | 10 |
| Swashbuckler | | 5 | 3 | -2 | 75 | 70 | 10 |
| White Ape | Girallon | 5 | 3 | -2 | 73 | 62 | 11 |
| Hippogriff | | 3 | 1 | -2 | 94 | 94 | 20 |
| Horse, Heavy | Warhorse | 3 | 1 | -2 | 96 | 99 | 23 |
| Gray Ooze | | 3 | 1 | -2 | 97 | 100 | 23 |
| Large Spider | Spider | 2 | 1/9 | -2 | 115 | 135 | 25 |
| Ram | Goat | 2 | 1/6 | -2 | 114 | 133 | 28 |
| Mule | | 2 | 1/4 | -2 | 113 | 129 | 17 |
| Zombie | | 2 | 1/2 | -2 | 112 | 114 | -2 |
| Gnoll | | 2 | 1/2 | -2 | 109 | 109 | 0 |
| Crocodile | | 2 | 1/2 | -2 | 110 | 111 | 15 |
| Giant Lizard | | 2 | 1/2 | -2 | 111 | 112 | 25 |
| Hydra, 10 Heads | Hydra | 18 | 17 | -1 | 25 | 29 | -4 |
| Gorgon | | 13 | 12 | -1 | 38 | 41 | -3 |
| Purple Worm | | 28 | 27 | -1 | 15 | 15 | 0 |
| Troglodyte | | 2 | 1 | -1 | 108 | 105 | 3 |
| Pixie | | 1 | 0 | -1 | 134 | 136 | 3 |
| Giant Toad | | 3 | 2 | -1 | 93 | 89 | 4 |
| Bugbear | | 3 | 2 | -1 | 92 | 86 | 6 |
| Displacer Beast | | 7 | 6 | -1 | 57 | 50 | 7 |
| Lizard Man | Lizardfolk | 2 | 1 | -1 | 105 | 96 | 9 |
| Ogre | | 4 | 3 | -1 | 83 | 73 | 10 |
| Gelatinous Cube | | 5 | 4 | -1 | 72 | 61 | 11 |
| Unicorn | | 5 | 4 | -1 | 71 | 53 | 18 |
| Homunculus | | 1 | 0 | -1 | 135 | 137 | 27 |
| Centaur | | 4 | 3 | -1 | 82 | 68 | 28 |
| Horse, Draft | Draft Horse | 2 | 1 | -1 | 106 | 103 | 29 |
| Giant Frog | | 2 | 1 | -1 | 107 | 104 | 30 |
| Giant Fire Beetle | | 1 | 1/6 | -1 | 133 | 134 | 29 |
| Jackal | | 1 | 1/6 | -1 | 132 | 132 | 30 |
| Giant Centipede | | 1 | 1/5 | -1 | 130 | 130 | 0 |
| Merman | Merfolk | 1 | 1/5 | -1 | 131 | 131 | 28 |
| Dervish | Cultist | 1 | 1/4 | -1 | 129 | 128 | 27 |
| Stirge | | 1 | 1/3 | -1 | 128 | 127 | 1 |
| Bandit | | 1 | 1/3 | -1 | 124 | 122 | 2 |
| Kobold | | 1 | 1/3 | -1 | 126 | 124 | 2 |
| Giant Rat | | 1 | 1/3 | -1 | 127 | 125 | 2 |
| Nomad | Tribal Warrior | 1 | 1/3 | -1 | 125 | 123 | 33 |
| Skeleton | | 1 | 1/2 | -1 | 121 | 117 | 4 |
| Dwarf | Guard | 1 | 1/2 | -1 | 122 | 118 | 4 |
| Goblin | | 1 | 1/2 | -1 | 123 | 119 | 4 |
| Wolf | | 1 | 1/2 | -1 | 119 | 113 | 6 |
| Horse, Light | Riding Horse | 1 | 1/2 | -1 | 120 | 116 | 27 |
| Umber Hulk | | 14 | 14 | 0 | 33 | 35 | -2 |
| Tyrannosaurus Rex | | 18 | 18 | 0 | 24 | 23 | 1 |
| Roc, Large | Roc | 24 | 24 | 0 | 19 | 16 | 3 |
| Hobgoblin | | 1 | 1 | 0 | 118 | 107 | 11 |
| Dire Wolf | | 2 | 2 | 0 | 104 | 88 | 16 |
| Orc | | 1 | 1 | 0 | 117 | 101 | 16 |
| Doppleganger | Doppelganger | 3 | 3 | 0 | 91 | 74 | 17 |
| Owl Bear | Owlbear | 5 | 5 | 0 | 70 | 52 | 18 |
| Wight | | 3 | 3 | 0 | 90 | 72 | 18 |
| Giant Hog | Giant Boar | 2 | 2 | 0 | 103 | 82 | 21 |
| Warrior | Bandit Captain | 2 | 2 | 0 | 102 | 80 | 22 |
| Hell Hound | | 4 | 4 | 0 | 81 | 58 | 23 |
| Rat | | 0 | 0 | 0 | 136 | 138 | 27 |
| Ogre Mage | Oni | 19 | 20 | 1 | 22 | 22 | 0 |
| Pegasus | | 2 | 3 | 1 | 101 | 75 | 23 |
| Werewolf | | 3 | 4 | 1 | 89 | 54 | 35 |
| Berserker | | 1 | 2 | 1 | 116 | 78 | 38 |
| Giant Fish | Giant Shark | 11 | 13 | 2 | 41 | 37 | 4 |
| Salamander | | 10 | 12 | 2 | 44 | 40 | 4 |
| Invisible Stalker | | 9 | 11 | 2 | 49 | 45 | 4 |
| Wererat | | 2 | 4 | 2 | 100 | 59 | 41 |
| Wraith | | 4 | 7 | 3 | 80 | 49 | 31 |
| Wyvern | | 9 | 13 | 4 | 48 | 38 | 10 |
| Giant Crocodile | | 5 | 9 | 4 | 69 | 48 | 21 |
| Weretiger | | 6 | 11 | 5 | 64 | 46 | 18 |
| Efreeti | | 13 | 20 | 7 | 37 | 21 | 16 |
| Troll | | 9 | 16 | 7 | 47 | 31 | 16 |
| Giant, Hill | | 8 | 15 | 7 | 56 | 34 | 22 |
| Wereboar | | 3 | 10 | 7 | 88 | 47 | 41 |
| Giant, Cloud | | 13 | 21 | 8 | 36 | 20 | 16 |
| Giant, Stone | | 9 | 17 | 8 | 46 | 28 | 18 |
| Giant, Frost | | 10 | 18 | 8 | 43 | 24 | 19 |
| Werebear | | 6 | 16 | 10 | 63 | 30 | 33 |
| Giant, Fire | | 11 | 22 | 11 | 40 | 19 | 21 |
| Dragon Turtle | | 34 | 46 | 12 | 11 | 6 | 5 |
| Giant, Storm | | 18 | 32 | 14 | 23 | 13 | 10 |
| Sea Monster, Large | Kraken | 35 | 50 | 15 | 10 | 2 | 8 |
| Dragon, Blue | Blue Dragon, Adult | 22 | 44 | 22 | 20 | 8 | 12 |
| Dragon, Red | Red Dragon, Adult | 27 | 50 | 23 | 16 | 3 | 13 |
| Dragon, Green | Green Dragon, Adult | 17 | 43 | 26 | 26 | 11 | 15 |
| Dragon, Black | Black Dragon, Adult | 15 | 43 | 28 | 30 | 9 | 21 |
| Balrog | Balor | 15 | 45 | 30 | 29 | 7 | 22 |
| Dragon, White | White Dragon, Adult | 13 | 43 | 30 | 35 | 10 | 25 |
| Black Pudding | | ? | 4 | ? | 5 | 60 | -55 |
| Titan | Empyrean | ? | 38 | ? | 4 | 12 | -8 |
| Ochre Jelly | | ? | 2 | ? | 137 | 90 | -4 |
| Beholder | | ? | 48 | ? | 2 | 4 | -2 |
| Lich | | ? | 46 | ? | 3 | 5 | -2 |
| Dragon, Gold | Gold Dragon, Adult | ? | 51 | ? | 1 | 1 | 0 |
| Rust Monster | | ? | 1/2 | ? | 138 | 108 | 10 |
