Thestrals are dark, winged horses that are so emaciated they look like living skeletons. They pull the carriages that takes the students to Hogwarts. You can only see one if you've witnessed a death. So to most they are invisible. Harry and Luna could see them, since they've seen death. After the battle, many have seen people die, this seeing the thestrals
Who hasn't? ... I mean uh.. what. I need to go, and uhhh... hide the body - I mean, uhhh.... I have to uhm... walk my fish. Alone. By myself. Without a dead body.
You never forget it when they just stop breathing. No Hollywood convulsions, no sudden rigidity, just there, then gone, 51 years of experiences, knowledge, family members loved, jobs held, debts owed, contests won, grandchild born nothing else matters after that final breath.
http://en.wikipedia.org/wiki/Matthew_Axelson