Bloodsucker is an enemy of the Teenage Mutant Ninja Turtles from the original Mirage comics. This creature was created partially by the blood of Raphael. It feeds on human blood. It sucks blood through its hands and has great strength and the ability to mutate things because of the mutagen from Raphael.
Bloodsucker was a normal leech until Donatello, Leonardo and Michelangelo decided to use it to get back at Raphael who had been disobeying Splinter and had stalled their training. Raphael was afraid of leeches so the other Turtles decided to force Raphael to let the leech suck on his blood. Bloodsucker sucked some of the mutagen out of Raphael and Raphael started to de-evolve back to a normal pet turtle. Bloodsucker started to evolve into a humanoid creature. Later Bloodsucker sucked the rest of Raphael's mutagen. The other Turtles decided to capture Bloodsucker so they could get the mutagen back to Raphael. Finally, they found Bloodsucker and started fighting it. During the battle, Raphael (who looked like a normal pet turtle at this point) started biting Bloodsucker thus draining the stolen mutagen back. After a duel between them, Raphael had gained back all his mutagen from Bloodsucker and was back to his former self. Bloodsucker had also transformed back to a normal leech which Raphael freed back to the river it had come from. But it still mildly craved turtle blood.
Bloodsucker's place in Mirage TMNT canon is uncertain given that, while the entire Guest Era (including the River trilogy) was declared non-canon by Peter Laird for having no direct Mirage staff involvement in its writing, some of the events of this story (notably Raphael's retro-mutation which Bloodsucker caused) are referenced in Sons of the Silent Age, a canon issue.
Bloodsucker's story was alluded to with the Creep in the 2012 TV series episode Within the Woods. Both mutants suck mutagen out of Raphael to enhance their own mutations while degrading Raphael's.
Since Rick Veitch never agreed to any work-for-hire contracts for his TMNT work, the rights to Bloodsucker (and all other characters from Veitch's TMNT stories) are in legal limbo.