Cucurbites were living spaceships that ran on blood. The Eighth Doctor considered them a myth, an "old wives' tale" predating known time.
They were indestructible spacecraft which ran on forever, filling their tanks with the gore of whole species. They either outlived their creators or sucked them dry, as well, and continued, eventually dying out when they were struck down by a strange plague.
The last known surviving Cucurbite plunged intot he magma of a newborn Earth millions of years in the past until she was expelled from a volcano and used Varney's ancestor to fuel itself. (COMIC: Tooth and Claw)
According to the Ninth Doctor, they were capable of drying entire species and worlds, swarming in millions. (COMIC: Monstrous Beauty)