Thestral

From Hpwiki

A Thestral is magical beast that looks more reptilian than horse-like. The black skin spans tight over the skeleton, and they seem to have no flesh between their skin and bones. The heads are dragonish, and the eyes are pupil-less and white. Thestrals are attracted by blood, and only people who have witnessed death can see them. Thanks to Hagrid, Hogwarts has a domesticated herd of Thestrals and uses them for carrying the stagecoaches that take the students from the Hogsmeade station of the Hogwarts Express to the school, and vice versa.

Thestrals are often thought to be terrible omens (or at least thought so by Professor Trelawny, and seem sinister but are actually helpful animals and not Dark creatures. Hagrid says they can defend themselves, like a dog would for example, but rarely do deliberate harm. Thestrals, like Hippogriffs, seem to understand (to an extent) what humans are saying: they are able to bring Harry and his friends to the Ministry from his command. Other varieities of winged horse include Abraxans and Aethonans.

Thestrals are described as "dangerous" by the Ministry Of Magic, and Dolores Umbridge, as Senior Undersecretary to the Minister, disaproves of Hagrid displaying them to his class.