Answer:
In the Tempest, Prospero keeps Caliban captured in a cave. By attempting to civilize him, he treats Caliban like an animal that needs to be tamed. In Act II, Scene II, Stefano and Trinculo refer to Caliban as 'a strange fish' which they consider selling as an attraction in England.
Whether or not it is just for the characters to threat Caliban like an animal depends on how we approach this issue. Caliban does not only physically resemble an animal - he behaves like one. He tried to take Miranda's innocence, which proves that he cannot control his basic drives. This makes him an animal rather than human. On the other hand, in his famous speech about the isle and its noises in Act III, Scene II, Caliban proves that he is capable of speaking in a beautiful way, which makes it unfair for the other characters to threat him like a savage.