Green’s Dictionary of Slang

a donkey, an ass.

1821		J. Burrowes Life in St George’s Fields 26: Miler a donkey.
1891		F.W. Carew Autobiog. of a Gipsey 140: I no longer drummed with my heels [...] Charlotte’s patriarchal white myla.
1900		Sporting Times 31 Mar. 2/1: If anybody will show me the hiding-place of the caitiff-wretch who told me Survivor was the best miler in the world and as good a plucked-’un as they make, the nark shall receive a presentation copy of ‘the great game’.