I said Wednesday, okay, I just didn't say which Wednesday.
Hippopotamus: The only previous mention of Hippos in D&D was in the Conjure Animals spell. With 8 Hit Dice and a damage range of 3-18, they’re a pretty tough monster, and they are said to be aggressive despite being herbivores. And in real life, they are responsible for a lot of human deaths. Any animal that can tear a low-level party to shreds like this one can isn't to be sneezed at.
Hobgoblin: Hobgoblins, which first appeared in OD&D Vol. 2, have not changed at all statistically. But they have been greatly expanded on, because the only description that OD&D gave for them was that they were bigger goblins with a bonus to morale. The Monster Manual gives them a tribal lifestyle (with such names as Leg Breakers, Marrow Suckers and Slow Killers), stats for leaders and chieftains, details of their lairs (including the use of carnivorous apes), what weapons they use, their physical description, and a hatred of elves. The militaristic bent that they will have in later editions isn’t really evident here, although they are illustrated in their traditional samurai-helmet look. They are said to always keep their weapons polished, which I guess is more military discipline than most humanoids show. But like most of the humanoid monsters (kobolds, goblins, orcs), at this point they’re basically the same monster with a different amount of Hit Dice.
Homonculous: This monster first appeared in Supplement I. Statistically there is a major change, as its Hit Dice drop from 6 to 2. That’s a big shift, but it makes sense to me given how small these guys are. Their Armor Class has imprived from 7 to 6, but that’s hardly enough to offset the hit point drop. Their bite attack now has a duration for the sleep that it causes. They now use the same saving throw values as their master ( and given what happens to the magic-user when their homonculous dies, a lot of PCs will be breathing a sigh if relief). They can also be controlled at a greater range than before – 48” as opposed to 36”. The process of creating a Homonculous hasn’t changed much. It still requires an alchemist, who needs 1-4 weeks and a pint of the magic-user’s blood. But the cost is now variable, so it could be more or less than the 1,000 gp it required before. And the M-U must now cast mending, mirror image and wizard eye, whereas before he didn’t need to cast any spells.
Horse: Horses first appeared in OD&D Vol. 2, and they’re a pretty important part of the game. The types detailed here are Draft, Heavy, Light, Medium, Pony, and Wild. The first four of those were in OD&D. Ponies have only been briefly mentioned in Swords & Spells, of all places. Wild Horses were in the wilderness encounter tables in Supplement III. Draft Horses gain an extra hit dice, and a damage range (which they didn’t have before). Heavy Horses have gotten slightly faster and gained a few hit points. Light Horses are unchanged, and Mediums have gained a solitary extra hit point. Ponies and Wild Horses get stats for the first time. It is mentioned that ponies can be trained for war, so 3rd Edition's war ponies do have some precedent. Figures are given for how much horses can carry, with one figure showing how much they can carry before being encumbered, and another being their maximum load. The first figure given matches closely to the maximum load figures for horses in OD&D, which means that a horse can now carry almost double what it was able to before.
Hydra: Hydras were one of the toughest monsters that first appeared in OD&D Vol. 2. The only statistical change they have here is that they are slightly slower. They are much the same as before, with the notable difference that each of their heads now has 8 hit points instead of 6 (an effect of using 1d8 for monster hit dice now). Otherwise it’s much the same.
The Lernaean Hydra appears for the first time. This is the version from the story of Hercules, the one that grows two heads every time you cut one off.
Pyrohydras also make their debut, being hydras with a fiery breath weapon.
Hyena: Hyenas first appeared in the wilderness encounter tables from Supplement III. They have more Hit Dice than I was expecting, but are unremarkable otherwise, having no special abilities to mark them out. Hyaenodons make their D&D debut here, but they are just extra big hyenas.