Seriously, I freaking love the show's creators. Obscure Aztec mythological creatures in a kids show? They just won all of my respect.

From wikipedia:
"The ahuizotl (or ahuitzotl) is a legendary creature in Aztec belief. It was described as dog-like, with hands capable of manipulation and an additional hand on its tail. The ahuizotl was feared due to its liking for human flesh, especially nails, eyes, and teeth. It was said to live in or near the water and uses the hand on the end of its tail to snatch its prey."
That's freaky even for the Aztecs.