bounty hunter

American Heritage® Dictionary of the English Language, Fourth Edition

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

Wiktionary

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

WordNet 3.0

1. n. a hunter who kills predatory wild animals in order to collect a bounty
2. n. someone who pursues fugitives or criminals for whom a reward is offered