White Ape (from Moldvay):
Move: 120' (40')
Att: 2 claws
Damage: 1d4 each
No. App: 1d6 (2d4)
I won't say these creatures are unheard of in games I've played or run, but they aren't run of the mill either. Because of their previously mentioned connection to cave men, I would also lump them in under "Lost World" creatures. They can be found in dungeons and in the wild as wandering monsters (though oddly not on the Lost World encounter table). They also seem to serve as the model for any large primate encounters (gorillas, etc.) as there are no other creatures of this type listed in the BX rulebooks.
As opponents, the apes are a bad time for starting or low level PCs, but not overwhelming. Their rock-throwing (1d6) and 2 claw attacks are dangerous, and 4 HD means you aren't putting one down in a single hit. On the other hand, their AC is pretty tame and they lack any really unusual abilities.
No, the thing I like most about these creatures (apart from the inestimable Mr. Otus' illustration) is that they are a great example of a relatively "normal" animal written up right. Low morale, keeping no treasure, and described as nocturnal vegetarian gatherers who will threaten before attacking. The idea that they have lost their coloration due to subterranean life is an interesting side note, but they are otherwise, well, just apes.
Not every creature has to be utterly mundane or completely fantastic (in the original sense of the word). The white ape is a nice balance between something normal and something just a little exotic.
And I like that.