Green’s Dictionary of Slang

longhorn n.

[the longhorn cattle that were found in Texas]

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

[US]A.H. Lewis Wolfville 109: Cherokee [...] is the modestest, decentest longhorn as ever shakes his antlers in Arizona.
[US]A. Adams Log of a Cowboy 195: Those old long horns, McNulta and Lovell, got us in with the crowd.
[US]C.A. Siringo Riata and Spurs 2: Hence a Texas long-horn kid was left alone among strangers.
[US]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.
[US]D. Shulman ‘Nicknames of States and Their Inhabitants’ in AS XXVII:3 185: Long Horns for Texans.
[US]Ragen & Finston World’s Toughest Prison 807: longhorn – A Texan.