Vampyroteuthis infernalis, not because it sucks blood but "because they have the ability to ‘cape’ themselves up with their membrane, which is black underside, looking just like a vampire wearing a black cape!"
The Lions Mane Jellyfish is the largest jellyfish in the world. They have been swimming in arctic waters since before the dinosaurs (over 650 million years ago) and are among some of the oldest surviving species in the world.
The largest can come in at about 6 meters and has tentacles over 50 meters long. Pretty amazing when you think these things have been swimming around for so long.