Green’s Dictionary of Slang

(UK, London) a monkey.

[1821		Egan Life in London (1869) 257: The dog-fancier [...] sidled up to the Swells, touching his castor, and thus addressed Jerry. ‘Mayhap, as how, Sir, you vou’d like to see a bit of fun to night vith this here little phenonmy Monkey. He has spoilt all the dogs that has fought him. Jacco Maccacco is nothing else but a hout-and-houter’].
1859		Hotten Dict. of Modern Sl. etc. 66: MURKARKER, a monkey.
1863		Story of a Lancashire Thief 9: As a lad, he remembered Jackey Macauco, the murkarker, which used to fight at the Westminster pit, and was chawed up at last by a whopping big bull terrier.
1873		Sl. Dict.