A Thestral is a breed of winged horses with a skeletal body, face with reptilian features, and wide, leathery wings that resemble a bat's. They are very rare, and are considered dangerous by the British Ministry of Magic. Thestrals are, undeservedly, known as omens of misfortune and aggression by many wizards because they are visible only to those who have witnessed death at least once or due to their somewhat grim, gaunt and ghostly appearance.
Due to Thestrals' classification as XXXX, only experienced wizards (or Rubeus Hagrid) should try to handle Thestrals. Breeding as well as owning these beasts may be discouraged or even illegal without Ministry consent; in fact, wizards that live in areas not protected against Muggles are forced by law to perform Disillusionment Charms on their Thestrals regularly.
A Thestral is also one possible corporeal form of the Patronus Charm.
Luna Lovegood: "They're called Thestrals. They're quite gentle, really... But people avoid them because they're a bit..."
Harry Potter: "Different. But why can't the others see them?"
Luna Lovegood: "They can only be seen by people who've seen death."
— Harry and Luna discussing Thestrals
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