D&D (edition-nonspecific): What monsters have you wanted to use but haven't?

[Malimar]

I've used pretty much every monster it's taken my fancy to use.

Even many of the various giant space hamsters that JAL_1138 cites! Including Woolly Rupert, the Tyrannohamstersaurus of Ill Omen. (I couldn't find good 3.PF stats for him, so I just slapped the Kaiju template on the Giant Hamster.) Also, Miniature Giant Space Hamsters (indistinguishable from regular hamsters) infested the party's spelljammer.

I've especially used the classics, the mind flayer, the aboleth, the beholder, the flail snail...

Though come to think of it, I've never actually used a beholder in combat. Gibbetyorb the beholder was the lawyer in a court case the Rakshasa Mafia brought against one party, and I've got an insane beholder running an underground trading post, but nobody's ever fought one. One party fought a gauth, once, but never a true beholder.

The one that's been low-key niggling at me lately that I haven't used but kind of want to is the Wolf In Sheep's Clothing, which I'll probably throw in somewhere sometime soon. Also, flumphs.