Halfling: Yes, I think at this point we can safely declare the use of the term ‘hobbit’ in D&D to be dead and buried. Halflings first appeared in OD&D Vol. 1, but they did not appear in Vol. 2, making this the first time that they get their own monster entry (they didn’t even get one in the Holmes Basic Set). They get the usual treatment for humanoids, with descriptions of their arms and armour, as well as the capabilities of their leaders. Their special abilities remain much the same, although their accuracy with slings now also applies to bows as well.
Of most significance is the introduction of the Halfling sub-races, Hairfoots, Tallfellows and Stouts. Hairfoots are the standard variety of halfling. Tallfellows, as their names would suggest, are taller and slimmer than regular hobbits, and sometimes pal around with elves. If they are strong enough, they can attain 5th or 6th level as Fighters (remembering that regular Halflings can only reach 4th). Stouts are the opposite, being smaller and friendlier with dwarves. A Stout with 18 Strength can reach 5th level as a fighter, and they all get infravision.
Harpies: Harpies first appeared in Supplement I. Whereas before they were said to have the lower bodies of eagles, now they have the lower bodies of vultures (which I guess is more evil-sounding). Their special abilities are the same, being a song that draws in any victim that fails a save, and a touch that charms them. They torture and eat their charmed victims, but get this: ‘What they do not want, they foul with excrement’. That is hardcore Chaotic Evil behaviour.
Hell Hound: Hell Hounds are fire-breathing dogs that first appeared in Supplement I. They have a couple of statistical changes. Their Hit Dice used to range from 3 to 7, but now it is 4 to 7. Their damage range has also been increased, from 1-6 to 1-10. We learn that they are not native to the Prime Material Plane, though it’s not specified where they do come from. (I think Hell is a pretty safe bet.) Their breath weapon now gets a range of 1”, but it still does a measly 1 point of damage per Hit Dice. But now that I think of it, they have no limit to how often they can use it, and it seems to me that they can do so even in the same round they use their bite attack, so that is actually pretty good. Their stealth and heightened senses are also better defined now, with actual numbers to back them up.
Herd Animal: Herd Animals first showed up as a result on the Wilderness Encounter Tables in Supplement III. It’s a wide-ranging category that includes such things as reindeer, oxen, giraffes, and antelopes. They’re not usually aggressive, but if they stampede your character you will be killed instantly, so don’t go pissing them off. Still it’s hard to take that picture of the dudes fleeing a lone rampaging giraffe at all seriously.
Hippocampus: It’s a new monster, but don’t get too excited, because this is pretty much just a bigger, smarter seahorse. Tritons ride them, and that’s about the only thing here of interest.
Hippogriff: Hippogriffs (just like griffons, but with horse parts instead of lion parts) first appeared in OD&D Vol. 2. They have a very minor statistical change, with their Hit Dice increasing from 3+1 to 3+3. They also can have treasure in a lair, which they didn’t have before. They don’t get much more detail than they already had, except for the ubiquitous open market prices for their eggs.