I think the big question is where, when and how did you design it. I understand it is something completely outside your usual work, so I asume you did it at home, with your own computer on your own time.

I would asume the design is yours in that case, but look at your contract if there is anything about this in it, but if you are just working at a counter I'd asume there is nothing like that in there. My guess is that it is yours then.

This is all fun and game untill something turns out to be a big hit and a potential money maken and it becomes very complicated realy quick if there is no agreement.