From the venerable OED: Maverick. A. n.    1. N. Amer. An unbranded calf or yearling.

1867 in J. G. McCoy Hist. Sketches Cattle Trade (1940) 83 The term maverick which was formerly applied to unbranded yearlings is now applied to every calf which can be separated from the mother cow. 1870 J. C. DUVAL Adventures Big-Foot Wallace xxviii. 163 A party of five Tonkawa warriors came within half a mile of my ranch, and in broad daylight killed one of my fattest ‘mavericks’. 1887 F. FRANCIS Saddle & Mocassin 172 Nowadays you don’t dare to clap a brand on a mavorick even. 1942 S. KENNEDY Palmetto Country 223 Most likely candidates for rustling are unbranded calves and cattle, called ‘mavericks’ in the West. 1974 G. JENKINS Bridge of Magpies xv. 229 Cowpunchers riding herd and trying to rope the most bloody-minded maverick that ever cut loose on the plains of Texas.