Breen

The Breen are an extraterrestrial humanoid species native to the ice planet Breen. Their true appearance remains unknown, as they have never been seen onscreen without their refrigeration suits, but the shape of their masks suggests that they have a snout. They are also notable for having no blood or other liquid running inside their bodies. Like the Ferengi, the Breen have a four-lobed brain.

As much as they are a reclusive, rarely seen race, they are also one of the most powerful military forces in the galaxy. Initially neutral, the Breen later joined the Cardassians and the Changelings in the a final war against the Federation.

Behind the scenes
Breen were first mentioned in "The Loss", a fourth season episode of Star Trek: The Next Generation that first aired in 1990. References to them were made in a number of other Next Generation and Star Trek: Voyager stories, but they did not appear until the 1996 fourth season Deep Space Nine episode "Indiscretion". They played a significant role in the final story arc of that series in 1999, during which much information about them was revealed.