Green’s Dictionary of Slang

(US) of a man, notably tough, esp. of a river-boatman; also used of a woman.

1809		W. Irving Hist. of N.Y. (1821) VI 235: The back-wood-men of Kentucky are styled half man, half horse, and half alligator by the settlers in Mississippi, and held accordingly in great respect and abhorrence.
1816		J.K. Paulding Letters from the South (1817) ii. 89: The great western road is travelled by the west country wagoners, some of whom, you know, are ‘half horse, half alligator,’ others ‘part earthquake, and a little steamboat.’.
1828		Richmond Whig (VA) 9 Dec. 2/5: A ‘salt river roarer.’ – One of those two-fisted backwoodsmen, ‘half horse, half alligator, and a little touched with the snapping turtle,’ went lately to see a caravan of wild beasts. After giving them a careful examination, he offered to bet the owner [...] that he could whip his lion in an open ring; and he might throw in all his monkeys, and let the zebra kick him occasionally during the fight.
1832		J.K. Paulding Westward Ho! I 83: A Kentucky boatman, who eveybody knows is amphibious, ‘half horse, half alligator’.
1837		Morn. Post (London) 28 Sept. 3/5: The genuine Yankee jury who sat on the body of a Kentuckian (hal;f-horse, half-alligator, with a touch of the whirlwind).
1838		T. Haliburton Clockmaker II 59: Whopping big fellows them, I tell you; half horse, half alligator, with a touch of the airthquake.
1846		T.B. Thorpe Mysteries of the Backwoods 183: ‘I’m a roaring earthquake in a fight,’ sung out one of the half-horse, half-alligator sort of fellows, ‘a real snorter of the universe.’.
1846		Hereford Times 11 July 6/2: Davy Crockett [...] was half-horse, half-alligator and a bit of a snapping turtle.
1848		N.Y. Herald 13 June 1/6: He is a tip top half-horse, half-alligator from Kentucky.
1855		‘Jonathan Jonah Goliah Bang’ in Diprose’s Comic Song Book 12: For every man is half a horse / And half an alligator.
1855		T. Haliburton Nature and Human Nature I 113: My! what a slashin’ large woman, that was; half horse, half alligator, with a cross of the mammoth in her.
1856		Wash. Sentinel (DC) 12 July 2/6: Six half-horse, half-alligator men from Kentucky [...] will put to flight all the loud-talking, spread-eagle Free-soilers and Abolitionists.
1860		Oregon Argus 13 Oct. n.p.: These half horse and half alligator sort of politicians are becoming a stench in the nostrils of the American people [DA].
1860		Inverness Courier 23 Aug. 8/1: The champion of the Church being bent on running what may be aptly termed a steeple-chase with the ‘half horse, half alligator’ of Sicily [i.e. Garibaldi].
1872		Schele De Vere Americanisms 347: The American who hesitates not to speak of himself as [...] an alligator, occasionally varying the phrase and making himself out to be half-horse half-alligator, in Kentucky, does not neglect the life on the waters.
1881		Derby Dly Teleg. 12 Dec. 4/2: We shall try to se the American as he sees himself [...] ‘half horse and half alligator’ with a dash of earthquake.
1890		Jasper Wkly Courier (IN) 17 Jan. 1/6: We’l show him that Kentucky boys / Are alligator horses / [...] / The flow’r of old Kentucky’s land, / Half horse, half alligator.
1891		Hants Teleg. 15 Aug. 12/7: Whoop! I’m a cantankerous old fighters [...] I’m half hoss and half alligator.
1910		Eve. Teleg. (Dundee) 7 July 4/4: Heenan, the American, who challenged him, described himself with justice as ‘half-horse, half-alligator and a bit of a snapping turtle’.
1936		C. Sandburg People, Yes 93: Mike Fink [...] half wild horse and half cock-eyed alligator.
1944		W. Blair Tall Tale America 64: I’m half-horse, half-alligator, and half-snapping turtle!
1955		Bad Day at Black Rock [movie script] I’m half horse half alligator.
1960		P. Oliver Blues Fell this Morning 117: Texas Alexander’s words recall those used by the keel-boatmen of the Mississippi, the Texas cowboys or the Ohio backwoodsmen who cried that they were ‘Half-horse and half-alligator, a little touched with the snapping turtle.’.
1980		Maledicta IV:2 (Winter) 237: The frontier braggart’s rootin’-tootin’, high-falutin’ ‘half-horse, half-alligator’ ringtailed rhetoric is in decline.