Green’s Dictionary of Slang

1. (Aus./US) of a horse, wild and unmanageable.

1903		Wide World Mag. Mar. 546: The whole Western country was scoured over for the wildest and most vicious ‘outlaw’ bronchos [sic] that could be found [DA].
1927		C.A. Siringo Riata and Spurs 140: He kept an outlaw horse with which to test new riders.
1949		N.Y. Times Book Rev. 13 Mar. 22/4: His brother Billy was thrown by an outlaw bronco [DA].

2. (W.I.) wild, barbarous, crude.

1927		H. Stafford Orange Grove 41: Her song it was outlaw (fine folks say improper) [DJE].
1955		cited in Cassidy & LePage Dict. Jam. Eng. (1980).