The Mro were large, crustacean-like creatures with four arms and legs and dome-shaped heads with eyes that face in all directions. They were capable of possessing armor and energy weapon emitters grown into their carapaces, integrated and powered by their own physiology through the use of genetic engineering.
The Mro were a dominant force in their time, feared for their physical and technological strength. They overpowered and exterminated most of their sentient Alpha Quadrant contemporaries, with the known exception of the Iconian Empire. Their civilization eventually collapsed when, for lack of other species to fight, the Mro turned on each other.
One Mro was kept in stasis in the Eridian Vault by the Federation Department of Temporal Investigations, presumably discovered by Starfleet and moved there to safeguard it against tampering. (DTI novel: The Collectors)