Green’s Dictionary of Slang

a pony, bred in India.

[1781		Hicky’s Bengal Gaz. 15-22 Dec. n.p.: TATOES Yeat’s Black H. Nincumpoop lost].
1854		Delhi Sketch Bk 1 Dec. 141/1: I was returning [...] in one of Brandon’s Dâk carriages, with one of his very lamest and most used up tats in the shafts.
1859		G.F. Atkinson Curry & Rice (3 edn) n.p.: [O]ur Griff became the happy possessor of a diminutive tat (as we designate tlie ponies in the East),.
1879		H. Hartigan Stray Leaves (2nd ser.) 338: Joe had the tat out in no time; the colonel mouned and rode off.
1888		Kipling ‘“Sleipner,” late “Thurdina”’ in Civil & Military Gaz. (1909) 140: ‘Anybody lost a tat?’ asked Marish cheerily.