Fig.: someone or something that is good at doing only one thing, that is limited to only one talent

Although Mike is primarily known as a breaststroke champion, he is far from being a one-trick pony, as he happens to hold the world record of 200-meter medley

Nonostante Mike sia conosciuto come un fuoriclasse del nuoto a rana, non è certamente il suo solo talento visto che detiene anche il record mondiale dei 200 misti

XY is a good actor, but he's a kind of one-trick pony: he's been playing the same role for 40 years on TV

XY è un bravo attore, ma sembra che abbia una sola freccia al suo arco; infatti, interpreta in TV lo stesso ruolo da 40 anni

The idiom comes from the circus world: a circus featuring a pony that has only been trained to perform one trick is not very entertaining. This US expression first appeared around the turn of the 20th century.