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1.
A zombie horse.
Thestrals are the zombies of the horse world.
by Draco/Harry September 23, 2012
 
2.
Magical creatures which are first mentioned in Harry Potter and the Order of the Phoenix when Harry sees them for the first time pulling the carriages they ride in from the Hogsmeade train station to Hogwarts. Harry sees them for the first time in this book because thestrals are only visible to humans who have seen someone die (Harry saw Cedric Diggory get murdered in The Goblet of Fire.)
Harry, Luna, and Neville can see thestrals.
by Erica A April 22, 2008
 
3.
A Dead Looking, Horse-like creature with wings. It can only be seen by people who have seen death, and appears in J.K. Rowling's Harry Potter series.
"They're called Thestrals."
- Luna Lovegood
by JayFlay November 27, 2007