Frank Jackson, a Princeton junior woke up at 6 a.m. Sunday when a horse knocked on his door. Jackson, who lives on the third floor of Walker Hall at Nassau, was disturbed.

Summoning police, the drowsy Princetonian helped lay planks down the stairs. Unfortunately, Walker Hall stair wells are too narrow for a horse to turn around in, so Jackson's visitor was backed down.

Officials doubt that the horse climbed the staird himself, and head proctor Michael Koplluor suspects it was the work of a prankster. "I don't think it's very funny," said one dour officer who was rousted from bed to help remove the beast.

Since few people in Princeton own horses, detectives had little trouble in finding this one's patron. He is Donald W. Griffin, Princeton alumni secretary. Griffin could not remember having left the horse anywhere besides his (the horse's) stall.