Thestrals

Thestrals are invisible to anyone who has not watched someone die and understood it. For years the carriages at the gates seem to move themselves. Then they have horses. The reveal is quiet, which suits the animal: they are gentle enough to ride, and associated with a kind of sight nobody wants as a qualification.

Canon fact: they live in the forest, they are visible only to some students, and the school uses them anyway. The reason a particular student can see them is a later-book grief, and this entry does not narrate it.