Rabbits and horses are hind gut fermenters. What it essentially means is that the Gastrintestinal Tract is run by fermentation In the hind gut aka the large intestine and cecum. Bacteria, Protozoa, and fungi are present in the cecum and break down feed stuff to create digestible nutrients. The Hind gut is essential in these animals and is the reason we must take extra caution when changing food. Any change in the diet of these animals can make the 'bugs' mad and can kill them off. This is where colic and stasis issues come into play.
Humans are not hind gut fermenters. They rely more on the enzymatic digestion of the stomach