Despite the movie itself not being well received, Megalon himself was very much well accepted by the fans for having a cool design and some decent fighting ability. It helps he has appeared in the Atari/Pipeworks Godzilla games as an easy to pickup and play fighter.
Gigan's status as one no doubt was the reason for his inclusion in this title.
Ho Yay: Goro and Hiroshi are often interpreted as a gay couple. That we're never given a solid explanation of how they even know each other doesn't help.
Godzilla's tail-slide against Megalon. So good, he does it TWICE, and it was later included as an unlockable move in Godzilla: The Game.
Narm / Narm Charm: Godzilla and Jet Jaguar using hand signals to communicate can be one or the other, depending on your mood.
Megalon and Gigan as well; they even do a high five/brofist at one point!
Rescued from the Scrappy Heap: Jet Jaguar in subsequent Godzilla fighting games where he's a fun character to use as well as his use in IDW's Rulers of the Earth comic.
The Scrappy: Rokuro was annoying enough in the original, but the dub provides him with perhaps the most awful, screechy voice of any Godzilla kid.
Serial Numbers Filed Off: A common rumor is that part of why Jet Jaguar didn't get off the ground was because the copyright owners of Ultraman took umbrage. If that's true, they must have buried the hatchet at some point.
Gigan got hit with this. In the new footage, Gigan's mouth and mandibles rarely move when he roars, and the mandibles are hanging at an angle the entire time like the internal supports are broken. He also never uses his belly buzzsaw, and the individual blades are all set at weird angles.
Look carefully and you'll find Gigan sucker-swipe in Godzilla from behind and slice open the top of his head with his buzzsaw. A spray of blood can even be seen before Godzilla falls down.
Visual Effects of Awesome: All that being said, a scene where Megalon destroys a dam is very impressive and obviously ate up a considerable chunk of the film's budget.