An ancient species who fought a war with the Promellians and left a deadly trap that still threatened ships a thousand years later.
The Menthars were an ancient species who fought a long war against the Promellians around a thousand years before the Enterprise era.
In Booby Trap, the Enterprise found a Promellian battle cruiser drifting in the Orelious system, still caught in a trap the Menthars had created with aceton assimilators, which drained a ship's energy and turned it into deadly radiation. Geordi La Forge, working with a holographic version of Leah Brahms, found the solution, and Picard flew the ship out by hand.