Green’s Dictionary of Slang

of a horse, like a carthorse, whether in build or breed.

1863		Reader 7 Nov. 529: The early illuminators’ variations of a few podgy fat-calved knights, and the carty fetlocked horses they should have ridden [OED].
1867		‘Stonehenge’ British Rural Sports 451/2: She was well bred, but very strong and carty; and she could go a wonderful pace for her shape.
1889		Dead Bird (Sydney) 27 July 7/2: [used of a human] Had he he kept going ever so little a bit harder, the ‘carty’ Foy would not only have not beaten him, but would never have caught him.