"Ravens are harder than frogs."
-- Ron's attempt to explain his difficulty casting Silencio on a raven (OP18)
The raven is a large black bird from the genus Corvus.
Although ravens do not appear to have any actual magical ability, but they have often been featured in many magic/paranormal stories. They have some eerie, haunting charm, perhaps just in their glossy black feathers, and Rowling chose to hint to that, but still keeping them out of the main picture.
Rowena Ravenclaw lent her name to the House she founded. Despite that name, the mascot of the House is not the raven as one might expect but rather the eagle.