Emil Zola was the author responsible for the Naturalist movement in literature. As member of the branch of the Realist movement, Zola believed that nature was far more powerful than man. Essentially, Zola desired to show that, regardless of what man did, nature would always prove to be far more powerful.
Therefore, the significance of the animals within the novel proves that nature is superior to mankind. While some would regard the horse, confined to the mines, as weak, the horse's ability to transform into a steadfast being able to survive life in the mines (far longer than many of the men who worked the mines).
Numerous times, throughout the novel, the mine (the central conflict) is presented as a beast. The mine devours the men each day, swallowing them whole. The only reason the men and women working in the mines are able to ascend the shafts is because the mine "allows" them. Again, this shows Zola's denotation of the power of nature over man.
Essentially, Zola's belief in nature's power becomes evident based upon the insertion of the animals within the novel. In the end, the mine (personified through its comparison to a beast) is the most destructive force. Nothing, regardless of what the characters try, can bring the mine down.