From the Dictionary of Nautical, University, Gypsy and Other Vulgar Tongues first published in 1859.
Moko: a name given by sportsmen to pheasants killed by mistake in partridge shooting during September, before the pheasant shooting comes in. They pull out their tails, and roundly assert they are no pheasants at all, but Mokos.
(What does Moko taste like? A lot like pheasant I’d imagine.)