Words like 'monkey', 'pony', 'ton' and so on are used by East End villains and Cockneys to denote numbers - ton is one hundred for example. Examples abound in popular culture (The Krays, Only Fools and Horses, Guy Ritchie movies). Where do these terms come from? And how could the words possibly connect to the amounts?
monkey = five hundred pounds (£500). Probably London slang from the early 1800s. Origin unknown. Like the 'pony' meaning £25, it is suggested by some that the association derives from Indian rupee banknotes featuring the animal.
The origin of ton supposedly comes from the fact that a ton is a measurement of 100 cubic feet of capacity (for storage, loading, etc).
That link above gives a good summary of what's known about British slang money words and expressions. I will just say that I'm "semi-Cockney" myself. Like most Brits, I know the value of a pony and a monkey, but I've seldom if ever heard them used except "facetiously". Ton, on the other hand, is relatively common (though not so common as the US import grand).