He is a one-trick pony who only plays one song.
one-trick pony
idiomA person or group noteworthy for only a single achievement, skill, or characteristic.
The director refused to hire the actor, fearing he was just a one-trick pony who could only play villains.
While the author achieved fame with her debut novel, critics dismissed her as a one-trick pony unable to replicate that initial success in her later works.
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dog and pony show
Any presentation or display that is overly contrived or intricate, while lacking real substance.
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flog a dead pony
To attempt to get more out of something that cannot give more; to attempt to arouse fresh interest in something that is either hopeless or already settled.
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pony in the barn
An exciting and real prospect, something to be legitimately excited about.
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shanks' pony
One's feet or legs, regarded as a means of transport.