D is for Dogfight (2012)

A man, adorned with a dog tag reading "Buddy", enters an underground fighting ring against a dog. The two have a vicious fight as various onlookers cheer them on. As the dog bites the man's neck, he shouts "Buddy!", causing the dog to stop. The dog suddenly turns on his handler, and lost dog posters are shown revealing that the dog previously belonged to the man. The dog and the man team up on the handler and presumably kill him.