Green’s Dictionary of Slang

a man on horseback with a woman riding side-saddle behind him.

1761		N.Y. Mercury 16 Feb. n.p.: This monster is larger than an Elephant, of a very uncommon shape, having three Heads, eight Legs, three Fundaments, two Male Members, and one Female Pudendum on the Rump. It is of various Colours, very beautiful, and makes a Noise like the conjunction of two or three Voices. It is held unlawful to kill it, and is said to live to a great Age. The Canadians could not give it a Name, ’till a very old Indian Sachem said, He remembered to have seen one when he was a boy, and his Father called it a gormagunt.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Gormagon. A monster with six eyes, three mouths, four arms, eight legs, five on one side and three on the other, three arses, two tarses, and a **** [i.e. cunt] upon its back; a man on horseback, with a woman behind him.
1791		G. Gambado Horsemanship n.p.: The art of riding before a lady on a double horse, vulgarly termed a la gormagon.
1806		Edin. Review Oct. 108: His disconcertment at finding no ink till he is directed to dip his pen into the tail of as ‘gormagon’.
1811		Lex. Balatronicum.