Large black bird from the genus Corvus.
Ravens were for sale in the Magical Menagerie (PA4).
Ravens, along with bullfrogs, are also used in OoP in Charms Class when practicing Silencing Charms.
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.