Why not browse a little bit? Asking questions like these is kinda silly considering "unique" has about 8 zillion meanings. Do you want something not many people keep? Something rare in nature? not many people keep common carp in an aquarium because they are huge ugly messy creatures but would be unique in a sence... there are award winning line bred guppy strains that are obviously unique.
You have no idea size or anything other than you want unique?