Native Planet: Breen

The Breen live on Breen and speak Breen - though this is electronically encoded by their helmets, which along with the rest of their suits generate an arctic environment inside. They have never been seen outside their suits.

The breen were mentioned many times on Deep Space Nine before they were actually seen, often as the butt of a joke. Ex. Yelgrun the Vorta says, "and I thought the Breen were annoying."

Breen article on Wikipedia