I always watched Mr. Ed. I read years ago they put peanut butter on the roof of his mouth to make him move his lips like that. I don't know if that was true or not. Allen Young was great in the show. I don't remember seeing him in to many other shows after that.
I remember seeing him in the H.G. Welles movie, "The Time Machine." (I am referring to the 1960 version.) He actually played two roles in that movie; one was a scientist with a Scottish accent and in the other role, he was the son of the Scottish scientist. Check this YouTube clip out at the 1:03 mark: