Thouls: half-troll, half-hobgoblin, half-ghoul. Yes, that is three halves. Thouls don't care about fractions. You wouldn't either, if you looked like a hobgoblin, regenerated like a troll, and paralyzed people like a ghoul.
The story goes that the thoul got its start with a typographical error in "Temple of the Frog", a mini-module contained in the original D&D supplement, Blackmoor. Apparently it was supposed to say "ghoul", but the T is right there next to the G, and just like that, the thoul was born.
The thoul got an official entry in the Moldvay-penned Basic Rules in 1981, and popped up again in Mentzer's 1983 "red box" set and 1991's Rules Cyclopedia hardcover. Thouls graduated to AD&D 2nd edition in the Monstrous Compendium: Mystara Appendix, and most recently showed up in the third-party D&D 3.5 accessory Dave Arneson's Blackmoor.
Anyway, as far as I can tell, the poor thoul has never gotten much love in terms of detail. Gygax never wrote about them, to my knowledge, so we'll never know if their hide is deep russet or burnt umber, or if they prefer tunics of dirty brown or mustard, or how many thouls in a given settlement will be leaders with an extra hit die, or whatever. All we know is that they're a "magical crossbreed" (so I guess they weren't born from a typical troll-hobgoblin-ghoul menage a trois), they're not undead, they "reproduce normally", and that they usually hang out with hobgoblins. (Incidentally, another old favorite of mine, the carnivorous ape, also was usually found with hobgoblins, and also has mysteriously disappeared. Clearly, hanging out with hobgoblins is hazardous.)
Anyway, that's the thoul: 150% monster, 150% awesome. So look out, skanky Clyde Caldwell princess lady. Thouls can paralyze people, they reproduce normally, and they're a-comin' for you.
Now I just need to get to work on statting these guys up.
(Apologies to Scott over at World of Thool for the pun.)