When I first saw that the Slann/Frog/Anurid had been renamed "Cuatl Lord" in the 0.10.0 SA book, I was a little bummed..."Cuatl" seems to come from the Nahuatl word for "Snake", and why would our big frog be called a Snake Lord? But on further reflection, I thought that "Snake Lord" made a good title for a great and wise leader. And using animals to denote rank or importance has precedent in the Aztec and Mayan cultures. I'm a little fuzzy on the details these days, but jaguars and eagles were both associated with victory in battle, hence the "Jaguar Warriors" and "Eagle Warriors" that you may have heard of. Warriors earned these ranks through success in combat. In iconography, a depiction of a battle would often show who the victorious side was by portraying them with jaguar-like or eagle-like characteristics. So I like the title "Cuatl" for the great casters, because it expresses a culture in which snakes are associated with power, leadership, wisdom, or something along those lines, and the Slann (ish) are therefore known as "Snake Lords" - not because they are snakes, but because that is their rank. Of course, maybe you who developed this meant to change them from frogs into snakes, in which case...never mind everything I just said. One more tidbit of animal imagery from the Aztec world that might inspire some fun sculpts or fluff: The Nahuatl (Aztec) believed that butterflies were the spirits of fallen warriors, returned to the world. It seems a little funny, butterflies being so delicate and all, but maybe they thought of that delicacy as an ethereal sort of nature. I don't know. But an image that occurs somewhat frequently in Aztec sculpture and iconography is of warriors with butterfly wings, or long noses like the butterfly's tongue thing. See here and here for imagery of jaguars and eagles, and here for an article about butterflies in mesoamerica.
I like your theory about this, and it's reasonable to think that, taking into account how similar "cuatl" and "coatl" are. About butterflies, i did not know that, but it is interesting. I've also heard a popular belief that says that releasing a butterfly grants a wish. Just a little little thing, "Nahuatl" is the language. The tribes and the people that spoke that language where "nahuas", and only part of them where the actual "mexicas" (aztecs), but there were other tribes that also used that language