one-trick pony
idiom

- someone or something that is only good for one particular purpose, or at doing one particular thing

Cambridge Dictionaries

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ORIGIN

The expression one-trick pony originates from the circus world and refers to a circus pony that comes out, performs one trick and then leaves. The idea that circus ponies perform only one trick is not that far from the truth.

One of the more common circus acts is having trained dogs leap onto and off trained ponies while they run in a circle. The ponies are not required to do much else but let the dogs use them as transport. This led to another idiom, “dog and pony show,” which refers to a display, demonstration or exhibition of something, such as when a person is trying to sell something.

Paul Simon of Simon & Garfunkel fame wrote a same-named song that relies on the figurative sense of a one-trick pony:

He’s a one-trick pony
One trick is all that horse can do
He does one trick only
It’s the principle source of his revenue
And when he steps into the spotlight
You can feel the heat of his heart
Come rising through

Pony is a 17th century word from the Scottish “powny” by way of the obsolete French “poulenet” (little foal), which is a diminutive of the Old French “poulain” (foal), and further from the Late Latin “pullanus” (the young from an animal).

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