Alan Young, who played...

...Wilbur Post, an architect who owned a talking horse in the 1960s sitcom Mister Ed, died at age 96.

According to his obit in the Times, Mr. Young:

...started the rumor that the crew got Ed to “talk” by coating his mouth with peanut butter. Actually, the crew would place a piece of nylon in Ed’s mouth; the horse would then try to remove it by moving his lips, giving the illusion that he was talking...

That's funny; when I was a kid my parents said they did it with "trick photography." (Everything they couldn't explain was done with "trick photography.")