Gantarooru was the monster of the week in Episode 3 of the original Gaiking Anime "The Mystery of Emperor Darius" and was mostly a last minute filler which appears at the end of the episode, as it strongly focus on a character; Zela #803.

_Abilities_

Outside of flight and strength, Gantarooru had Optic laser beams and Fire breath, as well was among the few Dark Horror Monsters piloted seen in the show. Gantarooru looked like a Turtle, with stubby limbs ending in claws, with even stubby legs, a Fish like face with ears, and had Gray, Dark Yellow and Purple in color scheme with green eyes. It was among the few Monsters in the series not destroy by Gaiking, but Zelan action.

_History_

"The Mystery of Emperor Darius" is quite a misleading title, as you get another information dump about the Zelans plight and in their predicament they created a Robot to be their God and Ruler, Darius, and he in turn he created his four Generals, all of which took control of the Planet and its population, converting portion of them into the Dark Knights and Dark Avian Corps Soldiers. All this info dump is done in about three minutes and that's the just 4 plus of the first 5:30 plus minutes of the Episode (discounting the almost minute and half of the intro). There's your mystery solve, motherfuckers.

Sherlock Holmes face palms to his falling death as he can't believe they use the word "Mystery" so subversively.

Riffing aside, to the real Meat and Potatoes of our little tale...

The focus of the story centers on Zela #803 – who like so many others, his life was upended by the Black Hole, in the outbreak of the catastrophe, his wife Rosa becoming one of the early victims. Now he takes his time raising his only daughter, Lhasa, in one of the underground cities built by the remaining survivors of the race, under the cold rule of their Robot God, Emperor Darius. While watching a Military Parade of the Dark Knights and Dark Avian Corps, three soldiers come to collect 803, as he has been selected to be inducted into the Dark Avian Corp; after refusing and a brief scuffle, he is taken away at gun point with Lhasa demanding they release him, but is instead shot to death by a Dark Knight, her body left abandon in the hallway of their apartment. Along with other Zelans, 803 is then taken to the Dark Avian Production facility, where he is process and converted into a Dark Avian Corps soldier, but driven by his hate of Darius for the death of Lhasa, he bypasses the Brainwashing and retains his will, now driven by his sole desire for revenge, holding firm to his musical locket necklace which contains a photo of his dead wife and now dead daughter, vowing to not allow the tragedy of Zela be spread onto other worlds.

803 then travels on Earth with the rest of the Army and is put with a strike team in another assassination attempt against Sanshiro; the later returns home to pick up his belongings to move to the base with Midori and Hachiro. On the way back to Lighthouse, the trio comes under attack by the fiends driving trucks, and while Sanshiro holds his ground - killing two of them with his psychically embedded Fire Miracle Pitch – Midori and Hachiro are taken hostage, forcing him to stand down; but before the killers could carry out the job, 803 rebels and kills the other Dark Avian, saving Sanshiro and the others. He then introduces himself to them, explaining his motives to join them, the hero taking notice of his pendant, but before any further talks can continue, a Scout vessel of the Dark Avian Corp appears, 803 allows Sanshiro and the others to escape while he distracts them, but is then captured and taken back to Zela for judgement. Instead of been executed however, 803 is taken to the Dark Knights and converted once more by order of the Emperor.

As Sanshiro and the others explain their fortuitous escape, much to the doubts of Pete, who reminds him the only thing they share with a Zelan is the Battlefield. Dr. Daimonji agrees while there may be peace loving Zelans, Darius would never allow such individuals to exist.

Indeed, 803 is reconverted and this time the Brain Washing is more "permanent" and returns to Earth to vowing to personally give his life to Destroy Gaiking; thus the Dark Army returns and unleashes attacks around Hokkaido's Daisetsu Mountains, in a ploy to lure Gaiking out. Daiku Maryu arrives on site and Gaiking is launched when in that moment the Dark Horror Kaiju Gantarooru attacks and goes after the Mecha firing its Optic Beams - to his horror, Sanshiro recognize (by voice and pendant) the Dark Knight Pilot as the Dark Avian Soldier who saved him before; demanding an explanation, 803 replies "All those that oppose to Emperor Darius, shall Die!". Both Robot and Beast fall to the ground lock in a holding in a grip, on impact Gaiking is tossed to the side of a mountain and a bleeding Sanshiro comes to realize that Pete's words held truth and takes the fight onto his once savior.

Gaiking jumps high avoiding Gantarooru fire projectile and fire Counter Punch, ripping the left arm of Gantarooru; as the beast crashes to the ground, the Locket comes lose from 803's neck and opens on impact, playing its musical tune; hearing it, 803 regains his memory and realizing what he has done, dashes towards Gaiking; Sanshiro, thinking is another attack, throws Counter Cross at the Beast's chest, however, 803's dash was to save Gaiking from been hit by a laser from one of the Grotestar ship and Gantarooru gets hit instead causing considerable damage to it. Looking at the photo his dead family, 803's last words are to his them that soon he will be with them, and he dies as the Robot Kaiju explodes, all that's left behind of him in the rising Mushroom Cloud is his damaged locket.

With tears in his eyes, Sanshiro, realizing 803 had saved him twice now, promises to destroy Darius in his stead.

Personal Note:

Wish 803 would had stick around a bit longer for a full character arc before giving him a Hero's Send off, but you know the troupe in these old anime - if you are a robot pilot for the bad guys with full story arc potential (be noble and honorable, or self center and treacherous), your lifespan is limited to one Episode.