Chewing on leather
From www.leather-dictionary.com - The Leather Dictionary
Animals chewing on leather
Some animals, such as horses or alpacas, are occasionally seen to be chewing leather, for example leather straps. This habit is interpreted by veterinarians as a way of relieving stress and that it is soothing for the animal. Furthermore, the chewing process - similar to humans chewing gum in - increases the salivary flow which, in turn, regulates the acid-base ratio in the mouth and prevents tooth decay.
For pet dogs there are extra leather bones that are vegetable tanned and chew bones that are formed from parchment, in these cases dried cowhide.
Video about a leather chewing alpaca.