Here's a breakdown:
* Gunsmoke: The main character, Matt Dillon, rode a horse named "Buttermilk."
* Bonanza: The Cartwright family patriarch, Ben Cartwright, rode a horse named "Buttermilk."
It's easy to confuse the two, especially since they were both popular shows airing around the same time. If you're thinking of a different Western series, please let me know!