Quiz: Remember the definition of mal de mer?
any person or group with only one discernable trait, talent, or area of expertise
That actor proved to be a one-trick pony.
from act in Cuffling Cousins Circus of mid-1800s
A person having a single accomplishment: As he proved during exquisite ballads and mid-tempo tunes, he's no one-trick pony/ For years, Twentieth Century Fund has been known to investors as a one-trick pony (1990s+)