Eh, I have to honest and say I'd put my foot down on the theory if we're counting every reptile. Reptiles are far more broader than a rodent or a canine (though I think counting a fox is also stretching it) and having a crocodile count just because it's in the same class as a snake makes it a lot more flimsy as snakes are very different from a crocodile.While I still hang on to the zodiac theory, I really don't think that they will strictly follow it. Rat being rodent-like. Dog being anything canine. So for snake, I believe it can be anything serpentine. Heck, I think it can be any reptile since there are no other reptilian animals in the zodiac besides the dragon and the snake.
If one day they do ox, I expect anything bovine. Or if they do horse, well, another Rapidash I guess.
If it's more serpentine then that's one thing, but if it becomes a crocodile or something different then I don't think it should be considered as a guideline.