Foxy

Foxy is a retired animatronic in Five Nights at Freddy's.

Foxy caused The Bite of '87
This is one of the most well known theory, again before the release of Five Nights at Freddy's 2. The main evidence here is the "SORRY! OUT OF ORDER!" sign in Pirate Cove. Also, he had very sharp teeth. He also had unstable, large jaw.

Foxy is a good guy
This theory simply about Foxy being different from the other animatronic. Foxy wanted to check on Mike, but his voice box malfunctioned and killed Mike via heart attack. He just felt guilty because of The Bite of '87, and changed his emotions. The reason why he's torn up is because he tried to protect someone from getting attcked.