Able to post pictures of that one in a group picture with it can you are others? Also, it could be a number of things. Give us some background? Age of fish? from the same spawn as any other fish in your group? How big is it right now? Water change routine. Size of tank. And anything else that my help provide more tailored feedback for you.
He could be genetics, it could be that you got a fish that should have been culled, could be stunted, a fish could also be delayed on a growth spurt and then all of a sudden start to catch up with the others, and there's other possibilities as well. Definitely those pictures would help.