Plasmavore

The Plasmavores were a race of blood-sucking near-humans. They were commonly criminals, murdering people across the galaxy and drinking their blood.

Biology
Though the Plasmavores were human-like in appearance, they were actually shapeshifters. By draining the blood of an individual, the Plasmavores could mimic the internal structure and genetic material of their victim. More extensive mimicry would require more feeding. (DW: Smith and Jones)

Plasmavores required blood to survive, as their bodies absorbed a lot of salt. (DW: Smith and Jones) While feeding, they produced exotic plasma energy. Though they needed to feed on blood, this blood-sucking could be become addictive, leading them to take risks and develop psychoses. (REF: Doctor Who: The Visual Dictionary)

History
The Plasmavores were known as a dangerous species, travelling the universe and looking for new species to feed on.

One Plasmavore, known as Florence Finnegan, fed on the Child Princess of Padrivole Regency 9 on a whim and was then hunted down by the Judoon. She escaped to Earth and hid in the Royal Hope Hospital, feeding on the Hospital's blood storage. She was protected by two Slabs. When the Judoon took the Hospital to the Moon, she fed on Dr. B. Stoker so that she could fool the Judoon scanners. She planned to kill everyone with an overpowered MRI scanner and then escape on the Judoon's Ship, but she also fed on the Tenth Doctor, giving her alien genetic material. This allowed the Judoon to recognize her as an alien and disintegrate her. (DW: Smith and Jones)