In John Steinbeck's Of Mice and Men, we follow the adventures of George Milton and Lennie Small as they struggle to find happiness as migrant workers in California during the early 1900s.
Because Lennie is a huge man of very simple intelligence, he is sometimes described as being like an animal. As far as I can determine, the term for this is "zoomorphism." This term is derived from a Greek word meaning "animal" (zo-) and "morphos", which means "form" or "shape." One of the definitions of zoomorphism is to give animal characteristics to a human being. The opposite of zoomorphism would be anthropomorphism, which gives human characteristics to animals.
One example of this occurs when Curley is attacking Lennie, who is not trying to defend himself:
Lennie covered his face with his huge paws and bleated with terror.
Note Steinbeck's choice of the words "paws" and "bleated." On one hand, giving Lennie paws makes him appear to be like a bear (or perhaps a dog). On the other hand, having him bleat causes Lennie to seem like a sheep. The term metaphor could also be used to describe the literary technique used here.