Why are some chips called pony in casinos?

'I'll bet'...you're talking about a British casino, where there are a number of slang terms for different betting amounts; a 'monkey' is 500 pounds, and a 25 pound bet is called a 'pony'.

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Casino chips are not referred to as "pony", I think the expression you are thinking of is "pony up" which means to pay up what is owed.

British Casino terms for a 'pony' also means 25.00 chip.This derives from the cockney money slang of the same ,pony=25.00.So, if you have a good win and you want to keep some of your winnings,you might want to ask for some Ponies (25.00 chips) with your win ,so you can stick them in yer pocket and keep some control of your loses/winnings.