Someone described as a one trick pony is a person who only has one particular skill or area of specialisation. Someone who is a one trick pony may not be very adaptable or be able to bring value to areas other than their realm of expertise. You might use this expression to say that even though someone has been successful or achieved a particular thing that you don't have confidence that they will be able to repeat their performance in another area.

1. The movie director made his name in science fiction, but his failed foray into the romantic comedy genre has seen critics label him a one-trick pony.
2. I thought John was a really good footballer but he's turned out to be a bit of a one trick pony he doesn't succeed when he's outside his comfort zone.

1. Even though we are known for our pens, I would like us to branch out into other types of stationary so that we're not just regarded as a one-trick pony.
2. Despite all the hype, the new phone has nothing special about it other than its triple-lens camera, making it something of a one-trick pony.