Jet-Jaguar

Jet-Jaguar was a robot created by the inventor Goro Ibuki and co-opted by the Seatopians to serve as a guide for their monster Megalon. Jet-Jaguar later regained his independence, and spontaneously programmed himself to grow to giant size in order to battle Megalon and Gigan. (They never bother to explain that miracle in detail, unsurprisingly.) Jet-Jaguar eventually helped Godzilla defeat the Seatopians' pair of monsters before shrinking back to normal size and rejoining his creator.