ONE TRICK PONY is pretty much a one trick pony, because that’s what it is.
That’s it, thank you, goodnight everyone!
Ok, a few more words: The single-minded trickiness was just how it turned out to be. Adding stuff is not always the solution.
About the saying, this is the most obvious idiom I’ve ever googled. TL;DR, circus! That’s all you need to know.
From ponies to fishies…