“Cow boy” (later “cowboy”) signified a tory partisan (pro-English) in the Revolutionary War (1770s). The name “cow boy” was used in New York and Massachusetts.
 
The first Texas “cow boy” (a man employed in grazing cattle) is cited from 1847. The early Texas cow-boys, like the Revolutionary cow-boys, were rough fellows with unsavory reputations.
   
   
(Oxford English Dictionary)
cow boy
1. A boy who tends cows.
1725 SWIFT Receipt to Stella, Justices o’ quorum, Their cow-boys bearing cloaks before ‘um. 1787 O’KEEFE Farmer, A flaxen-headed Cow Boy, As simple as may be. 1887 A. LANG Johnny Nut 1 A little cow-boy named Johnny Nut.
 
2. U.S. Hist. ‘A contemptuous appellation applied to some of the tory partisans of Westchester Co., New York, during the Revolutionary war, who were exceedingly barbarous in the treatment of their opponents who favored the American cause’ (Bartlett Dict. Amer.).
1775-83 THACHER Mil. Jrnl. (1823) 285 Banditti consisting of lawless villains within the British lines have received the names of Cow-boys and Skinners. 1825 J. NEAL Bro. Jonathan III. 290 Who knows but you are one o’ the tories yourself or one o’ the cowboys? 1857 W. IRVING Washington (1865) IV. ix. 109 A beautiful region..now almost desolated by the scourings of Skinners and Cow Boys.
 
3. a. In the western U.S.: A man employed to take care of grazing cattle on a ranch.
It is typical of the cow-boy that he does his work on horseback, and leads a hard rough life, which tends to make him rough and wild in character.
1849 J. S. JENKINS Hist. War U.S. & Mexico i. 52 The Mexican rancheros..ventured across the Rio Grande..but they were immediately attacked by the Texan ‘cow-boys’.
 
10 August 1776, Pennsylvania Evening Post (Philadelphia, PA), pg. 395:
BOSTON, August 2.
(...)
Every day our ears an eyes are saluted with drums beating, and colours flying for volunteers (as they call them) with Mr. Cowley at their head, with a dray and several barrels of beer on it, to entice the rabble; and, like the wide ocean, he receives all pick pockets, shoe boys, cleve boys, and cow boys with green boughs stuck up (to save the expence of cockades) in their hats, together with many fagots decorated in the same uniformo…
 
31 July 1783, Massachusetts Spy, or Worcester Gazette (Worcester, MA), pg. 3:
SPRINGFIELD, July 29.
We hear from Northampton, that on Thursday last an affray happened betwixt Mr. Easha Brown, an inhabitant of that place, and one Norton, late of Long-Island (said to be of that class of people, well known under the denomination of Cow Boys) in which Mr. Brown received several blows from the latter, which instantly put an end to his existence.
 
13 May 1814, Gleaner (Wilkes-Barre, PA), “Death of Major Andre” by Joshua Hett Smith,  pg. 4:
...he soon began to be more pleased, and in the most impressive manner entreated us not to proceed one inch further in the night, as it was very dangerous, for the Cow Boys had been out the preceding night, and had done much mischief, by carrying off cattle and some of the inhabitants as prisoners.
 
26 July 1847, Democratic Telegraph and Texas Register (Houston, TX), pg. 2:
CHARGE TO COW-BOYS.—The editor of the Huntsville Banner has lately been hunting for news among the “cow-boys” of Jefferson county, and among other queer items furnishes the following humorous report of the proceedings, on the last day of the session of the District Court held at Beaumont. (...) “For, believe me, when I inform you that there are other products of agriculture as important as sweet potatoes, and other vocations as profitable as herding cattle.”
 
October 1848, The United States Magazine and Democratic Review, “Aunt Beck: Or, the Texan Virago and the Tailor of Gotham,” pg. 322:
The old man was a cattle-driver, or “cow-boy,” as those men are and were termed who drove in the cattle of the Mexican rancheros of the Rio Grande border, either by stealth, or after plundering or murdering the herdsmen! They were, in short, considered as banditti before the revolution, and have been properly considered so since. This term “cow-boy” was even then—and still more emphatically, later—one name for many crimes; since those engaged in it were mostly outlaws confessedly, and if not so at the beginning, were always driven into outlawry by the harsh and stern contingencies of their pursuit, which, as it was in violation of all law, compelled them frequently into the most heinous crimes, to protect themselves against entailed consequences.