Isekai Beasts

An Isekai Beast is, like its name would imply, entities from another world. Isekai Beasts are both a reoccurring antagonist and the first source of conflict in Zi-O: Another Story. Despite their shared name however, Isekai Beasts aren't a race of Monsters and don't uniformly have a hierarchy nor do they necessarily have to be related to one another.

Furthermore, despite the name of "Beast", they do not have to be mindless or a form of monster since Humanoid Beings and even other-world Kamen Riders can be classified as an Isekai Beast.

Backstory
Isekai Beasts were not a regular inhabitant of the Universe and indeed, they only began appearing on Earth a few decades prior to the modern day and necessitated the creation and sustaining of Kamen Riders to prevent the extinction of Humanity.

Within the few decades the Isekai Beasts was present, they massively influenced and changed the culture of Earth and became ingrained in mythology; almost deified in some cases. A particularly powerful and intelligent Isekai Beast named Anak-Beros is a particularly worshipped entity by the Cults and Heretical for possessing enough strength to completely wipe out almost all 700 Kamen Riders worldwide in just a few days; possibly humanity's closest point to Extinction and accepting their fates before Anak-Beros suddenly disappeared.

Origins
Isekai Beasts enter the Universe via unstable rifts that connects to other Universes; bringing them over. It is of note that once transferred, these beings and entities usually can't go back to where they came from and therefore forces the Kamen Riders to deal with them as Peaceful Resolutions usually cannot occur.

These rifts can range greatly as many different causes can bring Isekai Beasts over to the present realm; be it Dimensional Walls, failed Magical Gates, Zips, Mirror Portals, or even Parallel Souls.

Classifications
Note: These classifications are based off of theoretical or recorded Data in-universe and as such, Members within the categories can be wrong/is subject to change.

Category Ruin
The Ruin Class of Isekai Beasts are the rarest and most powerful of the Beasts and like its name would imply, could bring ruin to the world as these have more than enough power to slaughter Kamen Riders with little difficulties.

Members:
 * Anak-Beros (Whereabouts unknown)
 * King Mantis (Off-Planet)
 * 2 others unknown

Category Genso Major
The Genso Major Category are a group of Isekai Beasts that are more powerful than even the strongest Riders known to Humanity and require groups of Riders in teams of 4 or more to harm it to any real extent.

Members:
 * Fate;Decay (Active)
 * Great Beast Shocker (Defeated)
 * Tourmaline Zircon (Defeated)
 * 9 others unknown

Category Genso Minor
The Genso Minor Category of Isekai Beasts are Beasts who require the strongest of the Riders to defeat and can defeat scores of average Riders if attempted to be fought without a proper plan. Usually, only Kamen Riders Omo, Zi-X, and Nether can take on one of these alone.

Members:
 * "Kamen Rider" Eternal (Shi Katsumi) (Active)
 * Ephemeral Megahex (Whereabouts unknown)
 * Chrysoberyl ("Brightly Gold Gem Kaiju") (Defeated)
 * Orb Baron (Defeated)
 * Strike Baron (Defeated)
 * 34 others unknown

Category Komorebi Major
The Komorebi Major is a category of Isekai Beasts that are just as powerful as Riders and if left to their own devices, is capable of wiping out entire cities without problem. Komorebi Minors who has particularly powerful or tenacious abilities is upgraded to this Category as well.

Members:
 * Undead Visor (Defeated)
 * Amora ("Unseeable Chironex") (Defeated)
 * 41 others unknown

Category Komorebi Minor
The Komorebi Minor category of Isekai Beast are significantly powerful and is a very dangerous foe even for Riders. At this level of strength, the world's strongest Nuclear Attacks are unable to damage it and is completely immune to most conventional forms of damage.

Members:
 * Silver Horn (Defeated)
 * Gold Horn (Whereabouts unknown)
 * Imbus (Javan Rhinoceros King) (Defeated)
 * Seven-Star Knight (Defeated)
 * Tohka ("Thunder God") (Active)
 * 99 others unknown

Category Destroyer Major
Destroyer Major is an immensely powerful Isekai Beast. Only the most powerful of Nuclear Strikes are able to defeat these beasts and it's these Beasts who forced Humanity's hand at creating Riders with immense powers far surpassing those of Nuclear Armaments.

Members:
 * Zugaikotsu Taberu (Literally "Skull Eat") (Defeated)
 * Cheng I Sao ("Thousand Undead Pirates") (Active)
 * 228 others unknown

Category Destroyer Minor
Category Destroyer Minor encompasses Isekai Beasts who are more powerful than the norm. Isekai Beasts who achieve sentience also coincidentally fall into this category more often than not. Extended military bombardments are usually capable of killing these Beasts with extensive collateral damage with these beasts usually being compared to walking Air Strikes in potency as they can easily wipe a small town off the face of the map if given the chance.

Members:
 * 419 unknown

Category Rust
The Rust category of Isekai Beast encompasses every Beast that does not fall under Destroyer Minor or higher. This is the largest category of Isekai Beast as this can range from ones who requires large-scale military operations to barely Superhuman-strength mindless drones.

Members:
 * 72,818 unknown