Breen

From FEInfobase

A highly aggressive race from the Alpha Quadrant. They form the Breen Confederacy. They sided with the Dominion during the Dominion War and almost turned the tide against the Federation. Their homeworld is reported to be a barren frozen planet. The Breen themselves wear refrigeration suits when leaving their vessels. Without them, they could very well die.

As a whole, they are a mystery as there are no records of anybody actually seeing what a Breen looks like under their suits. Their status after the Borg war is unknown.