I would use "which" (or "that") as a relative pronoun when describing animals, EXCEPT when you are writing a work of fiction in which the animals take on human qualities. E.g., (1) the tiger is a large feline that preys on other animals. But (2) Winnie the Pooh, who has many friends such as Tigger and Eeyore, loves honey just like any other bear!

You might also use "who" when you talk about a pet you own. For example, my dog, who loves to chew on things.