I is for Ixitxachitl

"Now what did I tell you about eating blowfish?" 

The supposedly-terrifying "Demon Rays" use slaves to tunnel out labyrinthine underwater cities in the coral. They're small, basically the size of a normal ray, with a poisonous barb on their tail, just like a stingray. In fact, they are stingrays. But with fangs and a silly name. You can get vampire ixitxachitl too, which seems like a slightly desperate addition to make them scarier. Best of all, we're told that for every 50 ixitxachitl there will also be a high priest and two guardians, all with Type U treasure ("magical items that can be used without hands"). Imagine that - a stingray wearing a gold chain. Or a cloak.

The Monster Manual helpfully informs us it's pronounced "ish-it-SHACH-itl", "icks-it-ZACH-it-ul", or "icks-it-zuh-chit-ul", which is basically an admission that you should say it however you damn please. Doesn't matter now though, as some bright spark eventually realised that ixitxachitl fulfil the same role as aboleths, and ditched them from the game.