The Thoul (perhaps my favorite monster) first appeared as a typo in one of the original books' Wandering Monster tables - it was supposed to be a ghoul. I believe it also appeared unstatted in one of the Monster & Treasure Assortments, but I could be wrong on that.
One of the guys at TSR (Gary Gygax? Tom Moldvay?) thought it was a good name for a monster and statted it up for his games. They made their first appearance in the 1981 Basic Book as a creature that looked just like a hobgoblin, but had some of the powers of ghouls and trolls. They were in each of the main BD&D rule books from then on. The 2e version of the monster was in the Mystara Monstrous Compendium.
Gary Gygax did a 3e version of the thoul (slightly re-named to tr'oul) in his Slayers Guide to the Undead. This version doesn't have the hobgoblin element and is basically a troll/ghoul combo.
R.A.