Bubble eye goldfish have a genetic mutation that accured naturally. But people have specifically bred that mutation/ deformity to create the big bubble eye strain of goldfish. It's a natural mutation but not the best for fish because it makes it hard for the fish to swim, their eyes don't sit in the right place, and the bubbles can pop and get infected.
Goldfish should look like a normal bronze common goldfish, long slim body with a short tail. All the fancy goldfish (bubble eyes, orandas, ranchus, fantails, comets, etc) have been made in the same way the bubble eye goldfish was made. A natural mutation that was bred and inbred to develop the strain.
Balloon fish are the same. A deformed fish appeared in a batch of fry. The fish was raised and bred to its parents and siblings. The offspring were bred to the parent with the deformity. And balloon fish were created.