In the Harry Potter universe, a Thestral is a breed of winged horse that is visible only to those who have witnessed death. They have a skeletal body, reptilian features, and wide bat-like wings, and are native to the British Isles and Ireland. Thestrals are used to pull the Hogwarts carriages, but only those who have seen death can see them.

Some key facts about Thestrals include:

• Appearance: Thestrals are skeletal creatures about the size of a horse, with a reptilian hide, white eyes, and leathery black wings. They are considered a breed of winged horse.

• Visibility: Thestrals are invisible except to those who have seen death. Harry Potter, for example, was unable to see Thestrals for years after his mother was killed in front of him, because he was barely out of babyhood when the murder happened, and he had been unable to comprehend his own loss.

• Symbolism: While Thestrals are somewhat intimidating in appearance, they are emblematic of a journey to another dimension, and reward all who trust them with faithfulness and obedience. Despite common belief, these rare and kindly creatures are meant to serve as positive symbols, especially when the themes of death and calamity become more prominent in the story.

• Rules for seeing a Thestral: Only those who have seen death can see Thestrals. It is not clear whether seeing the death of a non-human creature counts, or whether one must fully comprehend the concept of death to be able to see them.

Overall, Thestrals are fascinating creatures in the Harry Potter universe, with a unique connection to death and the afterlife.