Thestrals

"I mean, a dog'll bite if yeh bait it, won' it - but Thestrals have jus' got a bad reputation because o' the death thing - people used ter think they were bad omens, didn' they? Jus' didn' understand, did they?"
~Professor Rubeus Hagrid during a Care of Magical Creatures class.

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 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 (and fully accepted the concept) or due to their somewhat grim, gaunt and ghostly appearance.

Thestrals can be domesticated and mounted, so they are used as an alternative to brooms, Apparition and other methods of transportation. Once trained, they are very diligent and will quickly carry their owners wherever they wish to go.

Due to their classification as XXXX, only experienced wizards 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.

Harry Potter: "What are they?"

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 Potter and Luna Lovegood discussing Thestrals.

Physical appearance

Thestrals have quite a disturbing appearance and the wizards who are capable of seeing them often only describe these creatures as being sinister and spooky. This is because they are seen as having big, bony figures and their dragon-like faces which bear white, glittering eyes that lack both expression and pupils. Additionally, they are lured by the scent of blood.

Being a type of winged horse, most of their anatomy is identical to a horse, excluding their large wings that sprout from their back. Unlike the Abraxan, another breed of winged horses, Thestral's wings do not possess any feathers at all; they have vast, black and leathery wings that are more similar to those of bats.

Their fleshless, lustrous bodies are covered with a translucent and glossy coat. This smooth and dark skin is a bit slippery and so thin that Thestral's bones are clearly defined through the entire extension of their sleek bodies.

These eerie horses have long black manes, as well as a large tail, either with flowing black hair, like horses or ending in a tuft, like zebras.  Another distinction is their sharp fangs used to seize and slash their prey.

Nature

These creatures are seemingly rather dull, though Professor Rubeus Hagrid states that they are "dead clever", and, in fact, trained Thestrals are smart enough to understand their rider's words when they ask to travel to a specific location.

These magical creatures can be found in dark environments, and the forest is their natural habitat. They communicate with each other through a shrill and strange shriek that resembles some sort of monstrous bird.

They appear to be loyal creatures, able to discern a friend from an enemy. Thestrals would forcefully attack anyone or anything they see as a threat and, in the unusual case of domesticated Thestrals, any enemy of its owners.

In the Battle of Hogwarts, Hogwarts' trained flock of Thestrals cooperated with Buckbeak, the Hippogriff, to attack the Giants fighting for Voldemort. It's unclear whether or not wild herds can similarly cooperate with other species.

Diet

Thestrals are carnivorous animals and are attracted to the smell of blood. Professor Wilhelmina Grubbly-Plank mentions that they often attack birds. This indicates that, naturally, they hunt not only for food in the ground, but also pursue flying prey.

The Thestrals that live within the Hogwarts grounds, in the Forbidden Forest, are properly fed and well trained. They don't attack other creatures or students unless seriously disturbed.

Abilities

The Thestral tail hair is a powerful and tricky substance that can be mastered only by a witch or wizard capable of facing death. It should be noted that this substance can be used as a core in a wand's conception and it was used to create the most powerful wand known by wizards, the Elder Wand.

The most well known ability of these beasts is their invisibility to those who haven’t seen death. In other words, they are only visible to people who have seen someone dying and fully accepted, understood and internalized the concept.

Thestrals have an extraordinary sense of smell and will easily recognize the smell of blood and fresh flesh, even if the source of the scent is rather distanced.

They also have quite a useful sense of direction. The Thestral can understand exactly where their riders need to go. If their riders have a certain destination in mind, they only need to say the destination and the creature will diligently carry them to the intended location - much like owls do with letters.

These gentle, winged beasts are very capable and fast fliers and can travel long distances hardly beating their large wings. For example, in 1996, six members of the Hogwarts herd (Harry Potter, Hermione Granger, Ron Weasley, Ginny Weasley, Neville Longbottom, Luna Lovegood) were capable of flying from Hogwarts (Scotland) to the Ministry of Magic (London) in a brief amount of time. It is possible that they were faster than even the fastest broomstick, as when Harry rode his, he did not think he had ever moved so fast, and he was the owner of a top-of-the-line broomstick, the Firebolt.

Their powerful wings are capable of lifting, at least, the burden of two humans plus their own weight.

Interaction with humans

Even with all their useful abilities, Thestrals are rarely used as methods of transportation due to their reputation as omens of evil and their somewhat dreadful and even distasteful appearance.
When riding a Thestral, the traveller usually holds the creature's mane to ensure balance during the flight (or ride.) To aid the mounting, the wizards also place their legs behind the wing joints to provide safety.

Flying on the back of a Thestral during a long journey is frequently an unpleasant experience, particularly to those who dare riding them without seeing the creature. The high speed flight on an invisible steed can be terrifying. The wind will, eventually, cause a temporary deafness and will force the riders to close their eyes. It is often difficult to keep balance on their slick backs.

The Hogwarts herd is gentle towards humans, they react satisfactorily to caresses and avoid attacking owls. However, taking into consideration the Ministry of Magic classification as "dangerous", this behaviour may be exclusive to well-trained Thestrals,[6] or just mere prejudice from the Ministry.

Hogwarts herd

Hogwarts School of Witchcraft and Wizardry has a very loyal flock of Thestrals used mainly to pull the carriages that lead elder students from Hogsmeade Station to the gates of the Castle. To people who cannot see Thestrals, it appears that the carriages are autonomous. The herd at Hogwarts started with a male and five females. A number of them have been born since, beginning with one named Tenebrus, which is a special favourite of Hagrid's, the Hogwarts gamekeeper. Harry and a group of students flew Thestrals from Hogwarts to the Ministry of Magic in an attempt to rescue Sirius Black. They were also used by Albus Dumbledore, when he needed to travel but didn't care to Apparate.

Rubeus Hagrid, the trainer and breeder of this specific herd, strongly suspects that this is the only trained large group of Thestrals in the whole of Great Britain.

Harry Potter's encounters with Thestrals

Harry Potter first saw the Thestrals at Hogwarts in September of 1995, after having witnessed the murder of Cedric Diggory in June. Harry could not see them that June because he had not yet dealt with what he had witnessed. At first, he wonders why the supposedly horseless carriages are suddenly pulled by such sinister creatures when they are able to move on their own. He points the Thestrals out to Ron Weasley , and realises that Ron cannot see them. Sensing his desperation, Luna Lovegood assures him that she has always been able to see the horses and that he is just as sane as she is. Given Luna's odd habits and beliefs, this statement does not completely reassure Harry.

They were first identified by name by Professor Wilhelmina Grubbly-Plank, after Hedwig was found injured, when she mentioned that Thestrals sometimes go after birds. They were next mentioned by Hagrid in a Care of Magical Creatures class, where the students were told that they could be used as mounts, and introduced their navigation abilities.

In June of 1996, Harry, Hermione Granger, Ron , Ginny Weasley, Luna Lovegood, and Neville Longbottom flew to London atop the Thestrals in order to find Sirius Black, whom Harry believed to be in danger in the Ministry of Magic. (Hermione, Ron, and Ginny had trouble mounting their Thestrals for the ride since, at least to the three of them, the Thestrals were invisible).

The Order of the Phoenix made use of Thestrals in the summer of 1997 during the Battle of the Seven Potters. Six of the Order's members took Polyjuice Potion to disguise themselves as Harry, and then the "seven Potters" and their protectors fled 4 Privet Drive to safe houses of other members. Hermione Granger and Kingsley Shacklebolt flew on a Thestral, and so did Bill Weasley and Fleur Delacour, as Fleur doesn't like brooms.

During the Battle of Hogwarts, Thestrals were seen attacking Lord Voldemort's Giant soldiers from the air.

Nineteen years later, James Potter II teasingly warned his younger brother Albus about the Thestrals before they left for Hogwarts.

Known people who can see Thestrals

Rubeus Hagrid.  Hagrid witnessed several deaths over the course of his life, including his father, and could see Thestrals.

Neville Longbottom.  Neville mentioned being able to see Thestrals in a fifth-year Care of Magical Creatures class.

Luna Lovegood.  Luna witnessed her mother die during a magical experiment gone wrong when she was nine years old, circa 1990-1991, and thus could see Thestrals when she first came to Hogwarts.

Harry Potter.  Harry did not see Thestrals immediately after Cedric's death in June of 1995; it took a few months before he accepted this tragic event, and then was able to see Thestrals.

Severus Snape.  Snape could see Thestrals, presumably as a result of seeing people killed in the First and Second Wizarding Wars, as Dumbledore once asked him "how many men and women" he had "watched die," though Snape had replied that "lately only those whom I could not save."