Muggles' Guide to Harry Potter/Magic/Thestral

Overview
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.

Extended Description
Thestrals are invisible except to those who have seen death; Harry can see them, but because his friends can't, he is initially afraid that they are a sign that he is going mad. It is Thestrals that pull the (apparently horseless) Hogwarts carriages to and from the train station at the beginning and end of the school year. They are carnivorous, and are attracted to the scent of blood. They are strong flyers and can carry you anywhere you want to go, and they have a good sense of direction. Because of the relationship with death, it is widely believed that seeing a Thestral is a bad omen.

It is not until Hagrid teaches a class on Thestrals that Harry is reassured that these are real creatures and he is not losing his mind. Only four students in Harry's Care of Magical Creatures class can see the thestrals: Harry himself, Neville Longbottom, a "tall, stringy Slytherin" who the author has identified as Theodore Nott and Luna Lovegood. Luna Lovegood (who saw her mother die) can see them as well; but she is in Ravenclaw, and a different year, so she attends a different class. Hagrid is proud of having the largest domesticated herd of Thestrals in the UK.

The ability of Thestrals to fly, and their navigation ability, will prove invaluable to Harry when he, Ron, Hermione, Neville, Ginny, and Luna need to get to the Ministry of Magic. We will see them again briefly in Harry Potter and the Deathly Hallows.

Analysis
One of the things we are beginning to perceive, by the time that Thestrals are introduced, is that Wizards in general are every bit as afraid of death as Muggles are. This is somewhat perplexing, as Wizardkind can create and perceive Ghosts, where Muggles cannot, and one would think that ghosts prove the existence of life after death. (For purposes of these discussions, we will ignore the matter of Shades and Horcruxes, as those are extremely rare in the Magical world, so the average Wizard will not have seen them.) In Harry Potter and the Prisoner of Azkaban, we are told that there is an entire book written about Death Omens, and from Ron we learn how seriously the Wizarding world takes them. The association with death that is caused by the Thestrals' selective visibility makes them undeservedly one such omen.

The author has commented that her mothers' death, midway through writing the series, may have caused something of an additional preoccupation with that transition in the books. However, the main story line, involving Horcruxes as a means of cheating death, and the Deathly Hallows, perforce will involve itself with death.

Questions

 * 1) Neville's parents have not died, they are alive still, if insane. Who did Neville see die?
 * 2) Hagrid must also be able to see them, but he didn't see his father die: Hagrid told Harry that his father had died during Hagrid's first year. Who would he have seen die?
 * 3) Generally, what do those who devote their lives to Magical Beast research when they cannot see Thestrals (yet)?
 * 4) How come the Thestrals drawing the carriages at the end of Harry Potter and the Goblet of Fire were still invisible to Harry? Does it take some time for the fact that you've seen a death to set in before you can see them? For that matter, why does he not see them from the very first story, as presumably he had seen his mother die?
 * 5) Are Thestrals dark creatures?

Greater Picture
The question has come up that Harry should have been able to see the Thestrals when he rode the horseless carriages down to the train at the end of Harry Potter and the Goblet of Fire; after all, he had just seen Cedric Diggory killed. There are actually two answers to why he could not, one in the context of the books, one in the context of the author's plans.

In the context of the books, then, while Harry has seen Cedric Diggory die, at that point he is still in shock and has not yet completely internalized it; Cedric's death is not entirely real to him, and it is only after he has been brooding over it all summer that he accepts that Cedric is dead. Only then, when he fully believes that he has seen death, is he able to see the Thestrals. Did he not see death earlier, when Lord Voldemort killed his parents? No, in fact he did not; he was not fully aware of it, being only a baby, and it happened beyond the sides of his cot so he could not see it. While it could be argued that Voldemort would have to see him in order to kill him, and the spell rebound killing Voldemort would have been visible to Harry at the time, Voldemort is not really dead, is he? Did he not see Professor Quirrell die at the end of Harry Potter and the Philosopher's Stone? No, he didn't; he had lost consciousness by the time Quirrell actually died.

The author has stated that she also had the idea that having the Thestrals visible to Harry just before the end of the book would have left another puzzle for the readers, a very uncomfortable one in fact: what are these creatures, are they Dark? So she made the conscious decision to leave them invisible to Harry until the next book.

The above is a paraphrase of the author's own comments on the matter.

It would, of course, also be possible that Harry did see the Thestrals at the end of Harry Potter and the Goblet of Fire, only failed to take notice of them having other thoughts on his mind, and consequently the reader is also not told about them. We strongly doubt this, however, as Harry would have had to remain completely withdrawn from the world throughout the entire process of getting to the train station from Hogwarts, and the "horseless carriages" are specifically mentioned as arriving at Hogwarts as students of the three schools are preparing to depart for the summer.

The author has also stated that the magical core of the Elder Wand is a tail hair from a Thestral, "a powerful and tricky substance that can be mastered only by a witch or wizard capable of facing death."