Klingons

Although now one the Federation's many allies the alien warriors known as the Klingons were originally one of the chief antagonists of both the Star Trek movies and TV series - they remain a proud warrior-race but no longer actively assault human fleets as they had done in the past and many heroes in the later series were Klingon.

Klingons were also anagonists in Star Trek: Enterprise due to the series being a prequel to the other movies and series - thus taking place in a point of history were the Klingon empire was still hostile towards humanity.