If. I don't think they are; it would have been obvious to some scientist at one point or another, not to mention anyone who lives in close contact with them.

It seems more plausible to me that their apparent intelligence is another product of magic; that when you're talking to a snake, you're actually talking to a magic-induced AI of some kind that will, if you asked it to do something, control the snake afterwards to suit your purposes.

The laws of physics here are already AI-complete, so it doesn't seem like a large leap to me.