When teaching Harry how to cast a patronus, Harry asks what a patronus looks like. Lupin gives him a vague answer about patronuses being different for everyone.
"What does a Patronus look like?" said Harry curiously.
"Each one is unique to the wizard who conjures it."
Prisoner of Azkaban - page 196 - Bloomsbury - chapter 12, The Patronus
However, every patronus we see in the series is an animal, ordinary or magical. Why wouldn't Lupin say that patronuses take the forms of animals? Of course, it is never stated that patronuses can't be a non-animal, but it seems unlikely. Is it just because JKR wanted Harry's patronus to be a complete surprise at the end of the book?