Monica Lamprey was a Lamprey.
She and her brother entered our universe through a cave in 12th century England. They looked like green-skinned human children. Richard de Calne named them Julien and Dominique after his grandparents, and took care of them for several weeks.
Julien eventually became ill from being away from his home environment, his skin changing to a normal human colour. They went back to the cave through which they entered our reality, but it had been sealed by Shepherd Mullen. Before it could be opened again, Julien died. Dominique, enraged, killed two of the villagers and ran away.
Over the following centuries, she tried to find a way to return to her home in the Time Vortex. In the mid-1900s, she adopted the identity of Monica Pike, the supposed granddaughter of Doctor Stephen Pike. By then, she learned about Professor Joseph Tungard, who had discovered new atomic elements which could open a subspace rift back into the Vortex. She planned to use Helen Lamprey as a temporal lodestone to drain the chronon energy from many parallel universes and feed on it. She was stopped when multiple versions of the Sixth Doctor sacrificed their own chronon energy to ground her in one time and place, then continue feeding her until she was destroyed. (PROSE: Spiral Scratch)