Something I started wondering about:

What death did Hagrid see to allow him to see thestrals?

And was it his idea for them to pull the carriages, or have they been doing it for ages?

(if these were answered somewhere, i don't remember, would be nice to know)

Also, does the "seeing death" thing count as seeing an animal die, or does it have to be a person? What about a magical creature like a centaur (who would probably kill you if you called him an animal) or a house elf?