From LeakyPedia

Thestrals are magical creatures who carry with them a taboo, and for good reason - you can only see a thestral if you have witnessed death. Despite their morbid reputations, thestrals tend to be gentle, kind-hearted creatures.

Know people to have seen thestrals are: Luna Lovegood, Harry Potter, Neville Longbottom, and Rubeus Hagrid.