When Arthur Lubin, the will-known "animophile" (he was also responsible for Francis the mule and Rhubarb the cat), first approached comedian Alan Young with the notion of playing second banana to a talking horse, Young reacted in an understandable way. "Who," he inquired, a little disdainfully, "needs a talking horse?"

That was before he met Mister Ed.

This urbane seven-year-old palomino is indeed a character. Mister Ed is a fearful snob who, as his behavior frequently suggests, reads nothing less esoteric than Lawrence Durrell. At the same time he is a shameless sentimentalist capable of lifting his voice in teary-eyed adulation of the flag and/or his mother (an unsophisticated plow horse). Like Jack Benny he is full of delightful sham and pretense and is constantly being caught red-hoofed in some outrageous deception. For example, in one episode of the syndicated Mister Ed series, Young catches his secretly watching a TV Western, an art form for which he had previously expressed a great horselike disdain.

"I thought," sniffed Young, "you didn't like to watch people riding your relatives."

"I'm not really watching," replied Mister Ed. "I'm just waiting for Lennie Bernstein to come on."

In short, Mister Ed is a horse of parts. George Burns, who as part owner of the show supervises the writing, says, "We don't treat him like an animal but like an actor. And it comes out funny."

Lubin, who not only produces and directs the show but also created it (from magazine stories by Walter Brooks), says: "If you're going to have an animal talk, you must consider him as a human being. You must positively believe the horse talks. You must never say, 'Bring the horse in.' He's not a horse; he's Mister Ed."

Young himself falls easily in the habit of talking about Mister Ed as if he were - er, ah, well - real.

"Mister Ed turns a mean phrase - and he knows it," explains Young. "He's very fond of pizza. He can't understand why he can't be taken everywhere. Of course he's a terrible phony at times. In some ways he's like an Army buddy - only if you're real stuck will he help you out."

The actor who does the voice of Mister Ed (and who, incidentally, takes great joy in doing it) has so melded his own personality with that of Mister Ed that at times he seems part of the soundstage walls. Indeed, he wishes to remain anonymous - only partly out of his fear that identification with the voice of talking horse might prejudice his chances of getting more serious roles.

Mister Ed's trainer Les Hilton has the same air of hardly being there at all. In fact, it is surprising how quickly one forgets that the trainer is at all times using his top-secret signal system, guarded as zealously as a bombsight, to manipulate Mister Ed, controlling the movements of head, mouth and lips with uncanny accuracy. Our spies, however, determined this much: Mister Ed's mouth movements are controlled by nylon line. The horse appears to "speak" because he moves his mouth as a person might to clear the roof of the mouth of peanut butter.

Playing straight man to a horse doesn't seem to bother Alan Young. "I love it," he says. "It's a happy show, a funny show, a dream to work on. This despite the fact that they always print the horse's best take."

Mister Ed's success (next fall it moves to the CBS network, Sundays at 6:30 [ET]) is in large measure due to Young's suburb underplaying, expert timing an fine feeling for the comic possibilities of the horse. You've got to be logical," he explains, "or comedy goes right out the window."