WordNet (r) 3.0 (2006):

fall guy
    n 1: a person who is gullible and easy to take advantage of
         [syn: chump, fool, gull, mark, patsy, fall guy,
         sucker, soft touch, mug]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "fall guy":
   also-ran, babe, boob, booby, butt, chump, cinch, credulous person,
   cull, defeatee, duck, dupe, easy mark, easy pickings, fish, fool,
   game loser, goat, gobe-mouches, good loser, good sport, greener,
   greenhorn, greeny, gudgeon, gull, innocent, leadpipe cinch, loser,
   monkey, patsy, pigeon, plaything, prize sap, pushover, sap,
   saphead, scapegoat, schlemiel, sitting duck, sport, stooge, sucker,
   the vanquished, toy, trusting soul, underdog, victim,
   whipping boy