Lotus hit the nail on the head this time. That is definately NOT a cory cat. Agreeing with the others, and working with fish a considerable amount of time myself, I would agree its of the Synodontis family and more specifically I would classify that cat as
possibly being a Synodontis Eupterus. The fact that it's hiding a lot is very characteristic of that species as most tend to be light-shy. They sold that to you as a cory cat?
Time to find a LFS with more informed management.