Even the most powerful wizard cannot control what form their Patronus will take, and the silvery guardians are, in their nature, anti-darkness devices. This is why Dumbledore had the Order of the ...

Luna Lovegood's Patronus is a hare, and is first seen in Harry Potter and the Order of the Phoenix during the scene that Harry is teaching the members of Dumbledore's Army the Expecto Patronum charm.