Dr. Bruce is an original Mudsdale character created by Zourik, his first appearance is in the comic Time to Grow.

Dr. Bruce is an anthro male Mudsdale, with a brown and black body. The tip of his ponytail, the inside of his ears, his muzzle, and the tip of his tail are orange.
His hair and the upper part of his face are black.
His eyes, however, are red.

As his name suggests, he's a doctor who researches evolution. (He is Dylan's doctor and "supervised Bryan's performance"^[1] in the Time to Grow comic.)
Today, he is much better known for performing mpreg on his patients.