Keefer did know what his Patronus was, and he didn't know of anyone else that had the same animal. His was an okapi. Most people have never even heard of them. But Keefer was one-of-a-kind, so it makes sense that his Patronus would be, too.

He scribbled down some notes he knew about okapis.

Well, Keefer did try to blend in to the crowd around people he didn't know, so that definitely fit. But in his comfort zone, he was odd--though he preferred to think of it as unique. Keefer was definitely a rare breed, what with his constant skipping and knack for having unusual views.