The convicted alien bad guy, Boris the Animal (Jermaine Clement), provides the opposition to the Men in Black in MIB 3.
He breaks out of prison intent on getting his revenge on the agent who put him there, and took out his arm in the process: Agent K. Boris travels back in time and kills young K, effectively erasing him from the future and putting the earth population in mortal danger. His race was believed to be extinct; Boris was the last. Apparently those reports were wrong.
See Boris The Animal