Turned out that she only knew about the creature because of a book her sister has about various Mesoamerican peoples, but she was able to find the name, which she carefully spelled out for me over the phone:
Anyways, when I finally started researching in earnest, I turned to my handy-dandy Dictionary of Mythology and found... nothing. There is something wrong when a book as massive as that one doesn't have what I'm looking for. Panic was waiting to set in, sitting by the doorway that is my self control, rubbing its little hands together gleefully. (Great, now I've got an image in my head of a malicious-looking imp squatting beside my brain.)
Thankfully, before I had a chance to get overdramatic, I found an entry entitled "Ahuizotl":
"an Aztec water monster
"This monster is said to be half dog, half monkey, with an extra hand on the end of its tail. It was said to pull people into the water with this hand, killing them and eating only their eyes, nails and teeth."
Excuse me, but did that book just say that it's Aztec? It's the right monster, just the wrong culture. But, since it was my best bet, I enlisted the help of Google. Here's what I found:
Most say that the ahuizotl is part dog and part monkey, though some say that it's probably more like a giant otter. While the Dictionary does not mention this, most also attest to this disturbing creature having hands at the end of each foot as well as at the end of the tail. Interestingly enough, while the hand at the end of its tail is described as a human hand, the ones that serve as feet are described as racoon- or monkey-like.
The diet of the ahuizotl is also under slight contention: some agree with the Dictionary, and others say it will eat everything and that the eyes, nails and teeth are its favourites. Most say that, if it hasn't gotten enough to eat recently, it will cry like a baby so that it can nab those who came to help the poor wee babe.
It generally lives in lakes or rivers, where it can drown unsuspecting fishermen, well-meaning citizens or probably even curious children. Moral of the story: don't be a fisherman, concerned about wailing infants, or curious. Or the ahuizotl will get you, my pretties!
Really, compared to the Stromkarlen/Fossegim/Neck/Nøkken/Nix/Thing, the ahuizotl is refreshingly consistent. You can count on it 100% to kill you if you get too close, no matter how nice you are, and you're even fairly confident what your corpse is going to look like when it's through with its meal.
There's so much that could be done with this creature. I could give suggestions, but I feel like it would be somewhat superfluous. Besides, I can practically hear the question on everyone's minds by now:
"Why on earth did you think the ahuizotl was Mayan, Thea?"
After researching all these wonderful details, I found the book where I'd first read about our monster du jour, and it turns out that I just happened to have had a brain fart of some kind. Not only does the book (The Fantasy Encyclopedia -fantastic for pictures, but lean on info) mention the creature's name, an emperor who used it as his emblem, and it also says very, very clearly that the ahuizotl comes from Aztec stories.
Well, I'm only human!
Monsters and Beasts (this page also has some really neat pictures of the ahuizotl)
And here's a cool-looking book that apparently has a section on the ahuizotl: