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]