Imp: Imps are a new monster, a type of minor devil that often serves as the familiar to a lawful evil wizard or cleric. And as far as familiars go, these guys are just about the best. Their own abilities are formidable, including a poisonous tail, regeneration, invisibility and suggestion. They’re immune to normal weapons, can’t be harmed by fire, cold or electricity, and have a pretty high magic resistance in general. They can also polymorph into the form of a spider, raven, giant rat or a goat.
The real benefit of having one as a familiar comes from the powers they bestow on their master. If the imp is close enough, it confers its own regeneration and magic resistance. The master also operates at one experience level higher. Admittedly, if it’s out of range he’s one level lower, and if the imp gets killed he loses four levels, so there’s always an element of risk. The imp can also contact the lower planes like a Commune spell, and presumably has no chance of going insane from the experience. That really is a great familiar, so long as you take good care of it.
Intellect Devourer: Intellect Devourers first showed up in Supplement III. They have been altered only very little (the only change being a shift to 6+6 Hit Dice instead of 6), which probably has to do with how late into the game's development they were created. I would think that Gary had a really firm grip on how to create a good monster by 1976. See here for my original write-up.
Invisible Stalker: Invisible Stalkers debuted in OD&D Vol. I and II, as creatures summoned by the Invisible Stalker spell. They haven’t changed in concept here, and their stats are the same. We learn that they are from the Elemental Plane of Air (which I believe had been implied before, but never explicitly stated) and that they roam the Astral and Ethereal Planes (as had already been established). Nothing before was said about their invisibility making them harder to hit, but now anyone attacking one suffers a -2 penalty. Likewise, they now gain surprise on a 1d6 roll of 1-5, which was never stated before. They also get a 30% magic resistance they didn’t have previously. Invisible Stalkers still resent long, ongoing tasks, and now their tendency to pervert their master’s wishes has been given some game mechanics to go with it (a 1% cumulative chance of turning on their master per day of service).
Irish Deer: Irish Deer are the prehistoric variety, very large and with fierce antlers. They’re only aggressive during ‘rutting season’. The only problem I see with this creature is the name, there being no Ireland in the official World of Greyhawk. As is usual in such cases, given that the World of Greyhawk is explicitly an alternate Earth, I will say that these creatures originated from the area of the world that is in roughly the same place as Ireland.
Ixitxachitl: These monsters first appeared in Supplement II, and like most monsters from that book they’ve received an extensive overhaul. Their Armor Class has changed from 6 to 5, movement from 9 to 12, and Hit Dice from 2-1 to 1+1. Their single attack once did 3-18 damage, but has been reduced to 3-12. Conceptually they remain the same, being intelligent manta rays with a whole lot of cleric leaders, but their nature as philosophers has been lost (much to my disappointment). There are still vampiric ixitxachitl, but they are only encountered half as often, and they are no longer turned by holy symbols. Their claim to vampirism is that they can regenerate, and their touch drains levels (neither of which the monster could do before). So they just got a very big boost, because looking back at the vampiric ixitxachitl in Supplement III, all the vampirism did was allow it to be turned. It was more of a weakness than a strength.