* Bovine refers to cows and other cattle.
* Proctology is the medical specialty that deals with the rectum and anus.
While there are veterinarians who specialize in the health of cattle, they would be called bovine veterinarians or large animal veterinarians. They might treat a variety of ailments in cows, but not specifically those related to the rectum.
It's likely that you've encountered a playful or humorous reference to this term. The combination of "bovine" and "proctologist" is often used for a chuckle, as it sounds silly and unexpected.