In the final chapter as Lennie returns to the clearing and the pond, he does not fling himself down and drink in long gulps like a horse as he has done in Chapter 1. For, this time he is a hunted animal. He comes quietly.
He knelt down and drank, barely touching his lips to the water. When a little bird skttered over the dry leaves behind him, his head jerked up and he strained toward the sound with eyes and ears until he saw the bird, and then he dropped his head and drank again.
These described actions are much like an animal of prey who fears its predator is near.
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.
The examples given above seem to prove the point quite well. The moment in the text where Lennie crushes Curley's hand seems especially apt, as Lennie's aggression seems to be driven by his unthinking fear.
One could easily argue that this behavior is animalistic and/or instinctive.
Lennie is described as having 'bear-like' paws early in the text. This simile gives him the qualities of a wild and powerful beast. We see the devastating effect of his bear paw when he crushes Curley's hand.
Almost immediately after we are first introduced to Lennie, his behavior is described as follows when he approaches a pond:
[Lennie] 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 the comparisons of Lennie to an animal seem very explicit, not only in the way he drinks but in the sounds he makes as he drinks.
Lennie is like an animal because he is big and strong, but also childlike. He reminds me of big dogs who act like little dogs. As the saying goes, all big dogs think they are lap dogs and all lap dogs think they are guard dogs. Everyone is afraid of Lennie, but he's fairly harmless in intent, if not in effect.