Descriptions of Lennie can easily lead one to regard him as animal-like. Two excerpts from the text can be used to justify this characterization.
Behind him walked his opposite, a huge man, shapeless of face, with large, pale eyes, and wide, sloping shoulders; and he walked heavily, dragging his feet a little, the way a bear drags his paws.
His huge companion dropped his blankets and flung himself down and drank from the surface of the green pool; drank with long gulps, snorting into the water like a horse.
Here, Lennie is openly compared to both a bear and a horse.