Avengers: Celestial Quest #4 (2002)
It is documented that they have the capability of space travel, however little else is known of their race. The Plasmagens are cable of emitting highly powerful force bolt from their hands, and are also capable of flight. Primo was able to withstand the cold vacuum of space without life support. It is unrevealed if this was an ability granted to him by the Thanosi or a natural ability of his race.
Besides their flaming exterior, they have the usual humanoid appearance, of two eyes a nose and a mouth, two arms with hands attached to them, each hand has four fingers and an opposable thumb, two legs and feet.