In sports betting, a pony is British slang for a sum of £25. Please note that while this term isn’t really used outside the Commonwealth, it’s still fairly popular with some members of British online sports betting communities.
“Jake bet a pony on his favorite team again.”
The origins of this term aren’t really well known. One of the theories states that it came from British soldiers returning from India, where the 25 rupee note had a picture of a pony on it and that the saying was simply converted for sterling to mean £25. The soldiers also used the term monkey to refer to £500, since the 500 rupee note had a picture of a monkey on it.
Another theory states that the pony comes from the time when five pound notes were white, and the cost of horses or ponies along with a wedding carriage was approximately £25. Consequently, people used to say “I’ll pay for the pony in white” while referring to the color of the money and the wedding. Therefore, five white £5 notes became a pony, which roughly corresponded to the cost of a wedding.