Buck is a "Buckskin" horse. Distinguishing markings are it's buckskin color and dark legs and mane.
I read a story once that when Bonanza was cancelled, Lorne Green bought Buck and made sure he lived out his life on a ranch where he was well taken care of.
Interestingly, Marshall Dillon's horse, on "Gunsmoke", was also a Buckskin named Buck.