Green’s Dictionary of Slang

(US, mainly Western) a tough, Texan old-timer or cowboy; also attrib.

1897		A.H. Lewis Wolfville 109: Cherokee [...] is the modestest, decentest longhorn as ever shakes his antlers in Arizona.
1903		A. Adams Log of a Cowboy 195: Those old long horns, McNulta and Lovell, got us in with the crowd.
1927		C.A. Siringo Riata and Spurs 2: Hence a Texas long-horn kid was left alone among strangers.
1932		B.M. Harvey Me and Bad Eye and Slim 69: These Longhorns ought to of been put in the field artillery where there’s horses.
1952		D. Shulman ‘Nicknames of States and Their Inhabitants’ in AS XXVII:3 185: Long Horns for Texans.
1962		Ragen & Finston World’s Toughest Prison 807: longhorn – A Texan.