BoJack is an adult, male, grade/thoroughbred cross horse weighing over 1,200 pounds as revealed in the pilot episode.

He is around 6'7" feet (15.2 hands at the withers, 19.75 at the ears) tall.

He has brown (medium bay) fur and a black mane, with a white snippet on his snout that begins at his upper lip and ends underneath his eyes, a white diamond-shaped mark on his forehead, and a pink spot on his nose.
 
Human-like body parts. Human body, horse head. 