Reading from my old 2nd ed Monstrous Manual, there's an entire paragraph on the Ixitxachitl page:
"Ixitxachitl is both singular and plural; it is properly pronounced ish-it-SHACH-itl, though many refer to them as icks-itZACH-it-ul or even icks-it-zuh-chit-ul."
And that book was first published in 1993. Safe to say you're not the first to wonder... and they didn't even have the Net back then, so there must have been some pretty vocal confusion. (Come to think of it, I own the third printing from January, 1994. The pronunciation guide may be an appendage.)