mudder

from WordNet (r) 3.0 (2006)

mudder
    n 1: a racehorse that runs well on a muddy racetrack

from The Collaborative International Dictionary of English v.0.48

Mudder \Mud"der\, n.
   1. a racehorse that exhibits a better than usual performance
      when the racetrack is wet or muddy.
      [PJC]

   2. An athlete who performs better than his/her average under
      muddy conditions.
      [PJC]