rustler (n.)

"to emit soft, rapid sounds when in motion," late 14c. (implied in rustling "moving about noisily"), a word of uncertain origin, perhaps imitative (compare Middle Low German ruschen, Middle Dutch ruusscen, German rauschen "to rustle"). Related: Rustled; rustling.

The meaning "steal" (especially cattle) is attested by 1882, and is probably from earlier Western U.S. slang rustle "make, do, secure, etc. in a vigorous way" (1844), which is perhaps a separate word, compounded from rush and hustle. Compare sense developments in bustle (v.), hustle (v.), and compare rustler. To rustle up (transitive) in the general sense of "gather up, round up" is by 1896.