bounty hunter

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

• noun One who hunts predatory animals in order to collect a bounty.
• noun One who pursues a criminal or fugitive for whom a reward is offered.

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

• noun A person who catches criminals and brings them to the police in return for a reward.

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

• noun a hunter who kills predatory wild animals in order to collect a bounty
• noun someone who pursues fugitives or criminals for whom a reward is offered