bloodsucker

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An animal, such as a leech, that sucks blood.
• noun An extortionist or a blackmailer.
• noun A person who is intrusively or overly dependent upon another; a parasite.

from The Century Dictionary.

• noun Any animal that sucks blood, as a leech, a mosquito, etc.
• noun A name of a common agamoid East Indian lizard, Calotes versicolor, perhaps so called from the reddish hue of the throat, as it does not suck blood.
• noun A cruel or bloodthirsty man; hence, one who sucks the blood of or preys upon another; an extortioner; a sponger.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) Any animal that sucks blood; esp., the leech (Hirudo medicinalis), and related species.
• noun obsolete One who sheds blood; a cruel, bloodthirsty man; one guilty of bloodshed; a murderer.
• noun A hard and exacting master, landlord, or money lender; an extortioner.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who drinks the blood of others, especially by sucking blood through a puncture wound; a hemovore.
• noun By extension, any parasite.
• noun By extension, one who attempts to take as much from others as possible.
• noun A vampire.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun carnivorous or bloodsucking aquatic or terrestrial worms typically having a sucker at each end