This is, except the name of an Aztec king, in Mexican folklore, an animal. Bernardino de Sahagún, a Spanish Franciscan, described int he sixteenth century this "unheard-of animal" to be the size of a small dog, black and smooth. Feet and hands like a monkey and a long tail.
The creature lived in water and pulled passersby down into the watery depths where it devoured the victim's eyes, teeth and nails. The body of the victim showed no wounds, but were covered with bruises.