Everyone else has already defined this, but I would like to add that the thestrals demonstrate a dramatic shift in the theme of the book. They are symbolic of death, but death as a natural and misunderstood part of life. They foreshadow Sirius's demise.
In the first book, the horseless carriages are a whimsical icon of the magical world. It isn't until Harry sees what they really are in book 5 that we realize that Harry has not truly been seeing things as they really are in the wizarding world.
A thestral is the invilible pullers of the carriages that carry Hogwart's students to the castle. We find out in The Order of the Phoenix that they can be seen by those who have witnessed death. That is why Harry, Neville, and Luna can see them. They also provide transportation to the ministry for Harry and the gang in their attempt to save Sirius and the prophecy.
A thestral is a horse-like creature with white eyes, wings and leathery skins. They have a very thin, skeletal appearance, but one of the most interesting things about thestrals is that they are the creatures who pull the horseless carriages to and from Hogwarts from Hogsmeade and the beginning of the year. Perhaps the most interesting thing about the thestrals is that they are only visable to people who have witnessed death. Harry is able to see them in fifth year because he has witnessed Cedric's murder. They are also visable to Neville and Luna.
A thestral is one of the many magical creatures in the world of Harry Potter. This beautiful horse-like animal can only be seen by someone who has seen death (in Luna Lovegood's case, who is the first to show Harry thestrals, she saw her mum die). They greatly resemble a horse without skin, and they can fly. I recommend reading Fantastic Beasts and Where to Find Them for more information.
A thestral is a horse-like creature. They pull the carriages at Hogwarts. They are usually invisible to people. The only way a person can see them is if they've seen someone die. Its a rather grim element to the story when you realize that by book 7, all the students of Hogwarts can see them.
Threstrals were introduced by Hagrid in his Care of Magical Creatures Class in the fifth book of the series. They are winged, skeletal horses that can only be seen by people who have witnessed a death. During Hagrid's class, only Harry, Neville Longbottom, and an unnamed boy from Slytherin seemed to be able to see the creatures. Luna Lovegood can also see threstrals.
Also, threstrals pull the carriages that carry students from the Hogwarts Express to Hogwarts Castle every year. Harry was unable to see the threstrals and assumed that the carriages were horseless until he witnessed Cedric Diggory's death in his fourth year.
The threstrals played another important role in the fifth book in that they transported Harry, Ron, Hermione, Neville, Luna, and Ginny to the Department of Mysteries on their mission to save Sirius Black. It turned out, however, that Voldemort had shown Harry a fake vision about Sirius being in danger in order to lure him to the Department of Mysteries, where Harry would be able to extract a prophecy that Voldemort greatly coveted.
a big mystical horse They fly and are used to pull the carriages to Hogwarts each year.
A thestral is a horse-like creature. They are very thin. They pull the carriages at Hogwarts. Thestrals are only visible to people who have seen death which is why Harry, Luna and Neville can see them. Harry had witnessed Cedric's death, Luna had witnessed her mum's death and Neville had witnessed his grandad's death.
A thestral is a winged horse like creature. One can only see these creatures if they have witnessed death. They fly and are used to pull the horseless carriages to Hogwarts each year. They are invisible to all who have not been seen someone die. Harry and Luna both can see them as they have both seen someone die. Luna saw her mother and Harry saw Cedric.