The Collaborative International Dictionary of English v.0.48:

Rustler \Rus"tler\, n.
   1. One who, or that which, rustles.
      [1913 Webster]

   2. A bovine animal that can care for itself in any
      circumstances; also, an alert, energetic, driving person.
      [Slang, Western U.S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

rustler
    n 1: someone who steals livestock (especially cattle) [syn:
         rustler, cattle thief]