Seriously insert Bunny Booty jokes in the comment section below, cause that name is asking for it XD

Bun Bun was the Monster of the Week on Ep.35 of Grendizer and No.39 in the overall line and was the 2nd monster killed by Koji, but first with the introduction of his new fighter: Double Spazer.

Piloted by Commander Beto, it was unique for possessing a land mode in its vehicle form, with cresent moon shape wheels and driller in the front end to burrow its way on the underground, it still posses flight capabilities. It's robot mode was a mix of a beetle body with six legs, it could sprout winds for flight, sporting two green color bird like heads with moon crescent crests on top. It also had a secondary head with one in case the two main ones were destroy: this one was a dome with an eye and a curve spike on top. It color schem was Gray, Dark and light Purple, Pink and Black. Bun could fire beams from its eyes, which hitting a human could evaporate the victim. It could disperse a mist meant to deplete any area of oxygen, release land mines and fire red laser beam from its mouth. It could also fire missiles from the portholes on its shell wing, and the shell wings could serve to shield the robot from energy attacks. The horn on the secondary head fired electric beams.

Bun Bun and Beto arrive on Earth in a meteor, crashing into the water, and killed a human witness to the event. Afterwards, Bun dug its way underground as Planes assign by the observatory search the crash site. Beto reports to Gandal and is brief about its mission: to search his the hideout of Grendizer which likely lies underground using the known location Grendizer has appeared to them.

At the Observatory, Koji unveils the new fighter jet to Daisuke and Hikaru, The Double Spazer. At that moment, the group is inform of moving Seismic activity, it's path leading to Goro's school. While Daisuke takes off on Grendizer using one of the new routes to avoid detection, Hikaru rides back to get her father and both run to get Goro, who is in a baseball game. Grendizer arrives on site as the shaking begins, Duke ordering everyone to abandon school, Goro reuniting with his family. Duke his way underground with the Screw Crusher, Beto detects the incoming intruder and retreats, with Grendizer on his tail. Bun Bun escapes from the ground and into flight mode, Grendizer jumps onto the Spazer to go after it, but the Beast again goes underground, forcing Duke to disengage the Spazer and dig after him, Beto again resurface and this time Grendizer loses him, having pull this tactic to slow him down rather than fight him.

Beto continues his way underground off course, the quakes it causes almost bury the Makiba family (though they make it to the Observatory a bit later). Gandal calls him demanding answer, Beto informing someone was jamming the sensors with disruptive waves, a fact Zurin later confirms. Gandal orders Beto to due with the mission as he sees fit, and Bun Bun begins to dig straight to the path way of Route 5, Umon informs Duke and he flies back to confront the creature. Concern with the possibility Duke might not arrive on time, Umon orders to ready to blow up Route 5, with the staff pointing out such an action could have the lake flooding the hangar, but Umon replies that the secret needs to be kept at all cost. The staff waits on stand by for his Command.

Grendizer makes it to the coordinates at utmost speed and ejects, digging his way towards the beast, catching it on time, avoiding the destruction of the Hangar, with 500 meters distance. Beto tries to deplete the Oxygen but Grendizer destroys the weapon with Screw Punch. Beto retreats Grendizer goes after it, releasing land mines though these have no effect and Grendizer comes out unscathed. Beto tries to stop Grendizer from connecting from the Spazer, but Duke uses Double Harken, decapitating the heads. Beto loses control in direction to the Observatory, Duke inform Umon and the lab takes shelter, the Observation platform is lower into a bunker (with the Makiba's inside), while Koji boards the Double Spazer, though Umon tells him to wait: on cue, the Beast crashes onto the Observatory's towers, blocking Koji's launch. Duke uses hand beams, but Beto deflects them with the Wing Shields, hitting the base itself. Beto fires its electric beam (from the horn on the secondary head). Umon tells Duke to separate from Spazer and informs Koji to be on stand by.

Grendizer separates, landing in front of Bun, as the Spazer comes from behind, grabs the Beast and carries it away: Koji finally launches the Double Spazer and Grendizer hangs on to it, both going after the Bun Bun. The Spazer seperates from the Beast to allow the final assault on the enemy: Grendizer fire both Space Thunder and then Screw Crusher Punch, ripping through the monster. However, Koji deals the final blow with the Double Cutter blade, slicing the Beast and its pilot in half. An angry Gandal watching the fight, with Zurin replying that with a new weapon at their disposal, destroying it becomes imperative now. Umon finishes the episode with the plan to combine Grendizer and the Double Spazer.

Side Note: yeah, the damn thing is the Jet Scrander of the series. It's a cool neat plane though.