tl/dr: Mr Ed was a talking horse that had a sitcom in the '60s. It was still playing in syndication by the '80s, so it would be known to most people in the US at the time.
I don't mean to be old-splaining here, but familiarity with Mr. Ed helps. It was in syndication during the '80s, so I remember it as a hokey show with a pretty catchy theme song. The question of "how did they get that horse to move his lips" in time to the voiceovers was a valid question. (Answer: mostly peanut butter).
In the Larsenverse, however, the horse requires no voiceover. He just needs to be properly motivated.