Animal is a broad term in the concept of the Bestiary. The real dead giveaway that a Tatzlwyrm is of animal nature Francis is that their alignment is True neutral. It's said in the rules that animals like wolfs, pigs, snakes, raptors, Elk, etc have true neutral for their alignment because they act on instinct their is no malice in their actions or general goodness behind them.
That's why they would fall more under animal companions their also the right CR for it. They'd be a high level animal companion though. I mean animal companions is just a general term, as is the word for familiar it just depends. I mean for example Fluffy probably means more to Francis then other creatures and what not for example. I mean in general you take damage for Fluffy don't you.