The Collaborative International Dictionary of English v.0.48:

Raider \Raid"er\ (r[=a]d"[~e]r), n.
   One who engages in a raid. [U.S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

raider
    n 1: someone who takes spoils or plunder (as in war) [syn:
         plunderer, pillager, looter, spoiler, despoiler,
         raider, freebooter]
    2: a corporate investor who intends to take over a company by
       buying a controlling interest in its stock and installing new
       management

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "raider":
   aggressor, assailant, assailer, assaulter, attacker, depredator,
   despoiler, forager, forayer, freebooter, invader, looter, marauder,
   mugger, pillager, plunderer, rapparee, ravager, ravisher, reiver,
   rifler, sacker, spoiler, spoliator, wrecker