By JKR

Originally published on pottermore on Aug 10th 2015

Manifesting as black, skeletal, bat-winged horses, but invisible to all who have never been truly touched by death, Thestrals have a somewhat macabre reputation. In centuries past the sight of them was regarded as unlucky; they have been hunted and ill treated for many years, their true nature (which is kindly and gentle) being widely misunderstood. Thestrals are not marks of ill omen, nor (their spooky appearance notwithstanding) are they in any way threatening to humans, always allowing for the fright that the first sight of them tends to give the observer.