- Horoboros: Wild Fierce Monster (豪烈暴獣 Gōretsu Bō-jū)
- Metsuboros: Parasitic Devastation Monster (寄生破滅獣 Kisei Hametsu-ju)
Horoboros' name possibly comes from Horobosu (滅ぼすOuroboros, a circular symbol depicted a snake (sometimes a dragon), eating its own tail, symbolising the cyclic aspects of nature and life.lit. Destroy) and
Metsuboros' name is an alternative spelling of Horoboros, as Metsu is also derived from the kanji of "滅" in onyomi spelling.
Recoiling from his defeat as Ultraman Orb Dark, Makoto Aizen knocked over his R/B Crystal Collection Case in a rage, only to immediately regret it and pick his crystals up, "apologizing" to them along the way. However, halfway through recovering them, he noticed Horoboros' crystal and smiled evilly as he picked it up, realizing that it could be his next great weapon against the Ultramen.
Makoto summons this monster during the Minato family's day off while trying to evaluate the Ultramen brothers' performances in battle. Horoboros managed to resist all finishers thrown at it by the Ultramen and knock them out of transformation. Makoto took this opportunity to reclaim his Orb Ring NEO and transforms as he prepares to clean up his own mess.
Orb Dark seemingly flaunts the monster, trying to look "cool" for the people while he fights. However, Horoboros later pins him down and Orb Dark had to end it by pulling a cheap move, putting it to sleep and then finishing it off with the Dark Origium Beam. After its defeat, its Crystal was picked up by Saki Mitsurugi.
After the defeat of Bezelb, Saki summoned the monster using her own Gyro, and it evolved into its Bipedal form. Horoboros quickly defeated Orb Dark, Rosso and Blu, leaving all three of their human hosts critically injured before Saki returned it to its crystal. Later, Horoboros was summoned by Saki again and Rosso and Blu fought yet another losing fight against it until Asahi Minato gave the Orb Ring NEO to her brothers, which they used to turn the tides of battle and they finally finished off Horoboros via the Triple Origium Beam.
In yet another attempt to defeat Rosso and Blu for being 'fake Ultramen', Saki transforms into Horoboros who, at first, easily overcomes the brothers, partially because Rosso was hesitant on fighting after learning of Saki's past. The brothers tried to call upon Ultraman Ruebe, but they were once again denied of the transformaton because of Katsumi's hesitation. Seeing Horoboros approach Asahi and putting her in danger, Rosso and Blu are finally able to transform into Ruebe once more when the brothers are reminded of what is important; their family. Ruebe then defeated Horoboros but decided not to finish off Saki, holding off on firing the Ruebe Vortex Buster. Horoboros then turned back into Saki. Saki used Horoboros' Crystal to transform again when Kamisori Demaaga attacks but due to her injuries from her previous battle, Horoboros is no match for Kamisori Demaaga. The extradimensional monster leaves, but after that Horoboros collapses, too tired to continue on. Saki uses Horoboros' power for the last time when Kamisori Demaaga was consuming the Ray Lines' energy, but she was defeated once more.
- Designer: Masayuki Gotoh
- Suit actor: Shun Takanashi
- Originally, his debut was going to be in episode 11, according to the timeline of an early toy catalog.
- This is the first Kaiju to defeat an Ultra without using any special powers or outside influence whatsoever (to the point of reverting them back to a human).
- Although Red King seemingly accomplished the feat earlier in the series, he had the help of using Rosso's Splash Bomb against him.
- In its bipedal form, Horoboros' roar is occasionally mixed with that of Melba.
"When the golden bird is clouded at its peak, mighty Horoboros will awaken."
- ―Legend in Pacific Records
Horoboros is an ancient Kaiju that awakened once every 333 years, coinciding with a period where sunspots appeared, bringing destruction to the land. Ancient people used conch shell instruments such as Triton's trumpet to lull it back to sleep with a special tune. Horoboros' presence was recorded in the Pacific Records and became a legend in the Shishigaoka area of Ikoma, Nara Prefecture.
Years ago, Yuka Ohta heard of Horoboros from her great-grandmother. Being a curious young girl, she sneaked out at night into the forest and saw the beast, but nobody except her great-grandmother believed her. This event gave her the inspiration to start studying monsters.
In the present, the day of the 333rd year was approaching. Yuka brought Haruki Natsukawa to Shishigaoka along with her and set off to track down Horoboros' trails, hoping to stop it from awakening. They had found one of the ancient conch shells buried underground, but were not aware of its use. When the sunspots appeared, Horoboros emerged from the ground. Following a clue left in the Pacific Records, Yuka attempted to use the sound of waves to calm down Horoboros, only for it to backfire and cause it to rampage, destroying the city and upsetting her. Ultraman Z attempted to fight off the beast with Beta Smash and Gamma Future, but was unable to keep up with its strength. Eventually, Yuka found out the purpose of the conch shell and managed to calm down Horoboros.
Unfortunately, Shinya Kaburagi/Celebro had been nearby. Enraged at the loss of his Belial Medal, he threw the Gillvalis and Galactron MK2 Kaiju Medals over to Horoboros, forcibly transforming it into Metsuboros (メツボロス Metsuborosu). Having been enhanced with mechanical parts and a particle cannon, Z struggled to fight it. Yuka noticed that Metsuboros was in pain due to its modifications, and pleaded to Z to finish it off, since the conch shell's melody would no longer work. Z transformed into Delta Rise Claw, using the Beliarok to put down Metsuboros using the Deathcium Slash. Horoboros' soul ran towards the sky and disappeared.
- Metsuboros' Charged Particle Cannon is named after the anti-Zoid weapon of similar name. In fact, this was semi-acknowledged by episode 16's director, Tomonobu Koshi, who jokingly added that to combat Metsuboros' new attack, they need the help from Blade Liger.
- Although Metsuboros is a forced transformation by injection of Kaiju Medals, it is also accessible as a "combination" in the toy release of the Z Riser by including Horoboros, Galactron MK2 and Gillvalis Medals.
- Height: 44 m
- Length: 48 m
- Weight: 44,000 t
- Origin: Ayaka City
- Powers and Weapons
- Deflection: Horoboros is capable of deflecting beams such as the Sparion Beam with its paws. It also managed to deflect the Storm Shooting while breaking free of the Ground Coating's effects.
- Durability/Endurance: Horoboros is able to endure repeated finishing attacks from Rosso and Blu, including the Zettcium Beam and Gravity Slasher, and still continue fighting afterwards.
- Strength: Horoboros has plenty of brute strength, allowing it to manhandle both Rosso and Blu easily and break free from being covered in stone by Rosso's Ground Coating.
- Agility: Horoboros is capable of running at high speeds and performing acrobatic maneuvers in midair.
- Extraordinary Jumper: As a feline-themed monster, Horoboros demonstrated the ability to leap at an incredible height.
- Mid-air Kick: Horoboros leaps towards its target in mid-air at high speeds, performs a somersault of sorts, and kicks the foe towards the ground. This attack is strong enough to revert an Ultra back to their human host in an instant.
- Evolve: When summoned by Saki Mitsurugi, Horoboros evolved from its Quadrupedal form to its Bipedal form.
- Height: 56 m
- Weight: 44,000 t
- Origin: Ayaka City
- Maximum Running Speed: Mach 4.5
- Maximum Jump Height: 950 m
- Arm Strength: 198,000 t
- Grip Strength: 100,000 t
- Powers and Weapons
- Horoboro Claw (ホロボロクロー Horoboro Kurō): In exchange for both of its paws, Horoboros gains claws on each arm.
- Megante Crusher (メガンテクラッシャー Megante Kurasshā): Its claws can generate a wide purple energy slash, and can cause a fatal blow to opponent after it hits. When used at full power, Horoboros gains an aura.
- Claw Empowerment: Horoboros can generate an energy impulse to empower its Horoboro Claw and pierce through an Ultra Armor.
- Deflection: Like his paws in his Quadrupedal form, Horoborus can ultize it's claws to deflect incoming energy attacks.
- Strength: Horoboros retains its quadrupedal form's brute strength, able to lift two opponents at once simply by holding them by the neck.
- Endurance: Horoboros retains the same hide from its previous form, being capable of resisting standard finisher beams. It can also negate Orb Dark's paralyzing attack.
- Acceleration: Horoboros can accelerate to high speeds.
- Burrowing: Horoboros can burrow at least at moderate speed, and can jump out of the ground.
- Ultraman Z
- Gicante Thunder (ギカンテサンダー Gikante Sandā): Horoboros unleashes eruptions of lightning bolts from its body.
- Height: 56 m
- Weight: 50,000 t
- Origin: Shishigaoka Town
- Powers and Weapons
- Mechanical Claws: Gillvalis' arms give Metsuboros additional strength.
- Charged Particle Cannon (荷電粒子砲 Kaden Ryūshi-hō): A powerful energy cannon fired from the front of its face.
|Ultraman R/B Kaiju|
|Ultraman R/B||Grigio Bone | Cereza | Black King | Gargorgon | Red King | Guebasser | Mecha Gomora | Aribunta | Ultraman Orb Dark | Gomess (S) | Horoboros | Bezelb | Gubila | Grigio King | Neronga | Pigmon | Booska | Dada | Alien Zarab | Alien Mefilas | Alien Chibull | Alien Bado | Alien Pitt | Alien Nackle | King Joe | Grand King Megalos | Gomora | Kamisori Demaaga | Grigio Regina | Leugocyte|
|Ultraman R/B The Movie: Select! The Crystal of Bond||Bemstar | Gan-Q | Alien Pegassa Pega | Mecha Gomora | Snake Darkness | Pigmon | Grigio Regina|
|Ultraman R/B Novel: The Blue-Eyed Girl Whose Name is Gray||Bakubarba | Nero | Margodon | Grigio | Fearmonger | Alien Egon Ciel | Barrigator No. 1 and 2 | Leugocyte|