In chapter one of Steinbeck's Of Mice and Men, the author describes Lennie with animal imagery. He does so to suggest that Lennie is not only simple-minded like an animal but also has the strength of certain animals. In the second paragraph of the novel, Lennie is physically described:
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.
Lennie is heavy and slow, not only physically but mentally. He is also powerfully strong and, as George says, "a hell of a good worker." In the third paragraph he is compared to a horse:
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.
The horse comparison comes up later in the book when, in chapter five, the horses in the barn are described. Like Lennie, they are chained. Lennie is figuratively chained to repeating his mistakes over and over, just as the horses are chained to the slats in the barn and have no freedom.
A little later in chapter one, Lennie is also compared to a dog. Like a dog he is usually obedient to whatever George wants:
Slowly, like a terrier who doesn’t want to bring a ball to its master, Lennie approached, drew back, approached again. George snapped his fingers sharply, and at the sound Lennie laid the mouse in his hand.
He's not able to do anything himself, but with George's guidance he is able to get along without incident. It is when he is alone that he often gets in trouble, as in chapter five when he is alone with Curley's wife. When presented with a situation which might throw him into a panic he reverts to his animal urges, and when Curley's wife struggles he shakes her, breaking her neck.