Parasites of the Titans
Science fiction is rife with massive and megascale creatures, from the classic king of the monsters, Godzilla, to the newest incarnations in Pacific Rim and Attack on Titan. The concept of diminutive humans facing massive creatures is a popular theme. The biggest problem that comes from facing monsters on this scale is that there are typically only one, or a small number of the monsters to deal with. This is fine for horror, and pulp-fantasy but it runs into a problem in terms of RPGs. When the group of heroic PCs have to face off against the giant monster, they are in a target poor environment, and there is little if any room for supporting actions, or side stories, the sort of side quests that fill out games, and side story arcs that turn battles and conflicts into emotional events.
The giant monster genre doesn't work well with the design philosophy of the Cosmic Era. The only way to fight such giant monsters is through the use of giant mecha and warships, leaving all the other levels of conflict to sit and watch, or run for their lives. While this is likely to be true to life in many regards, the Cosmic Era is not about running away from monsters, it's about fighting them, with the whole bag of toys, but it's going to be nearly impossible for the parapsychic martial artist to tornado kick Cthulhu or Godzilla, no matter how good she is.
Behold, the Swarm
Just as the outer gods cast off astral debris that can metamorph into ravenous spawn, so too can the megafauna monsters and cosmic horror monsters inadvertently create it's own unique biozone, generated from it's own massive viscera and biology. The most natural and obvious type of creatures are going to be those that function in a parasitic manner.
The Parasite swarm creatures or roughly the size of a dog or average child, and can infest the nooks and crannies of a monster that weighs several hundred thousand tons with ease. Swarm creatures are human scale in terms on conflict and they ideally can be fought with firearms, martial arts, and organization. In a horror based setting, a few creatures of this scale can terrorize a group of unarmed or improvised armed civilians. In a action/adventure setting, these are cannon fodder for the PCs to warm up on, and in a tactical/military game, the medium and heavy gear has to be protected from the vermin, tanks weren't made to kill acid secreting ticks the size of large dogs.
Attributes: individually low, greatly magnified in numbers. One is a minor threat, a hundred is a massacre.
Abilities: minor, typically either a single passive ability, such as acidic secretions or nauseating smell, or a limited use activated ability such as a poison dart spit, poison bite, super leap, etc.
Attack Power: low, with minimal damage potential. Damage would be on par with a canine/wolf attack. Those with claws, spines, etc will obviously do more damage.
Defense: low, armor carapaces are thin enough to be fire-axed through, and should be vulnerable to even civilian grade and improvised weapons.
Example: Cloverfield 'Fleas'. The flea is a small creature, which is fast and agile, but can be killed easily with a bludgeoning weapon, pistol, or other firearm. It sacrifices armor, and hit points for speed. It's basic move set consists of tackle, and a special bite attack that injects a horrific enzyme that causes extreme bloating and the 'explosion' of the victim from the bite. Only a threat to unarmored civilians.
3. Aggro Spawn
The Aggro spawn is the bigger, meaner, older brother of the parasite swarm. These creatures are the size of humans, up to 2-3 times larger, placing them on scale with most power armors. Like the parasites, these creatures have been spawned from the detritus of their patron, but rather than functioning as pests, lurking in folds of flesh or 'whatever' the kaiju is made of, these are more autonomous, more aggressive, and better prepared to face the hostile world outside of their kaiju based biosphere.
The Aggro Spawn is intended to be a basic level fighting creature, typically multi-use, and in the form/purpose of the kaiju that spawned it. A good rule of thumb is that the larger to aggro, the fewer numbers they will appear in, and these spawn are predominantly designed for combat, functionally existing to spread the biozone of the kaiju, eliminating the spawn swarms of other kaiju, and those pesky humans who keep getting involved and in the way.
Abilities: One passive ability related to host, and an activated ability/special attack, also related to the host.
Attack Power: High, functioning on infantry/man to man damage scale, but they are by design, aggressors.
Defense: Average to low, Aggro spawn aren't intended to work as meat shields, fight defensive fights, or take hits. Their ability to dish it out should be more than they can take. This puts them in a disadvantage in ranged combat, which is really good for gun toting space marines.
Examples: The Xenomorph of the Alien franchise, adaptable, fast, dangerous with stabby tails, claws, hellacious bite, and acidic blood. The Hydralisk of the Star Craft franchise, fast, ranged spit dart attack, stabby and grumpy.
4. Tank Spawn
Counterpart to the Aggro Spawn, the Tank Spawn is large, and rather than being hostile and aggressive, the Tank Spawn is often lethargic, or even passive in nature. Tank spawn often serve another apparent 'primary purpose' such as the Tanker Bug in Starship Troopers, but at the end of the day, it is a slow moving, high armor high hit point enemy that soaks up damage, saps ammo and energy taking it down, and a number of them can be used to break through a fortified location. The Tank spawn also as it name suggests, functions on the vehicle damage scale, rather than the man to man scale.
Tank Spawn can function as specialists, such as mobile siege engines, having a special attack that makes them more dangerous. They can also be much heavier fighters, dealing massive damage and having very thick armor, but not being as fast or homicidally violent as aggro spawn.
Attributes: High, but agility and speed are low.
Abilities: several, the tank spawn can actually appear as a miniature version of the host kaiju. In a hive mind/swarm, the Tank is a focal point or node. It can have multiple special abilities, but again, typically slow in using them.
Attack Power: Either very high with a low rate of attack, or average to low.
Defense: The Tank Spawn is tough, requiring either squads of special forces to take out, or calling in air strikes, artillery attacks, or the attention of armored vehicles or light to medium mecha.
Example: The Tanker Bug from Starship Troopers. The Ultralisk from Star Craft, the Alien Queen from Aliens
5. Winged Spawn
I didn't know they could fly!
Giving things the ability to fly has been messing up the best laid plans for years. The winged spawn typically moves in the same circles as the Aggro Spawn, but does so from the air. Many of Lovecraft's horrors have wings, the Mi-Go, the Great Race, the Yith, and Cthulhu and his starspawn, they all have wings.
Like the Aggro, the Winged spawn is hostile and favors attack over defense. The main function of the Winged Spawn is that it forces those fighting the swarm to constantly look up, and to remove the normal protection afforded by flight capable vehicles. Evacuating via a helicopter, or other relatively slow vehicle can become much worse when the air is full of hostiles, and when bigger craft are available, watching the monsters splatter against the armor hull can be satisfying.
Attributes: average to high, winged creatures are going to be fast, and it isn't uncommon for them to be smarter than the average aggro as well.
Abilities: average, on par with a regular aggro spawn.
Attack Power: High, but often limited by their size. Aggro spawn can hit harder, but lack the speed and mobility of their winged kin.
Defense: Low, winged spawn tend to be fragile, though not nearly as fragile as pest spawn.
Examples: The monsters from Pitch Black, with their hammerheads and leathery wings. Mutalisks from Star Craft, and many of the large flying insects from The Mist.
6. Aquatic Spawn
One of the most consistently overlooked venues is underwater, which is a shame because 70% of the planet is water, and the most common terrain feature on the planet is water one mile deep. Aquatic spawn are a larger catch-all for what could be several entries. There can and likely should be aquatic aggro spawn, aquatic tank spawn, and aquatic swarm spawn, especially if the kaiju in question is a water based, or amphibious monster.
The main attribute of aquatic spawn is the ability to survive underwater, and the ability to function there with ease. It is unlikely that aquatic spawn are going to have ranged special attacks, and are much more likely to be more robust and stronger than their terrestrial counterparts.
Attributes: Average to High, water is dense and doesn't make room for weaklings. If you're going to be delicate, you better be dangerous as all get out.
Abilities: High, with most abilities being contact based, such as stingers, chemical jets and secretions, and the like.
Attack Power: High, and if something isn't physically strong, it's special attack will be.
Defense: Average to high
Examples: The creatures from The Abyss, Sharks, Killer Whales, Anything that lives in the abyssal zone of the ocean mixed with some G Cells.
7. Shapeshifting Spawn
The most dangerous creatures are those that can adapt quickly to their surroundings. Shapeshifting Spawn take this to the next level, and are able to reshape themselves to fit the need. This is a rare sort of spawn, typically relegated to exotic kaiju, such as those that are composed of hyperdimensional masses, appear of clouds of eyeballs, or manifest as a rotating ring of interlaced plasma celtic knots. These creatures tend to be powerful, threats in an of themselves, and not always organic in nature.
The most dangerous spawn are those that learn to become mimics, appearing as humans, or mundane things, be it livestock, or a massive insect creature that happens to resemble a main battle tank. These are intelligent infiltrators, and can do more damage than an army of aggro spawn.
Abilities: high to very high
Attack Power: Variable, sneaky shapeshifters and mimics are going to be less physically intimidating than the metamorphic offspring of a hyperdimensional horror.
Defense: variable, but typically average to high
Examples: Kowaru from Neon Genesis, he was technically a Kaiju, who just happened to be a 14 year old boy. The Transformers are technically this, assuming they are the spawn of Primus, aka Cybertron.
There was a fanfic of Neon Genesis Evangelion that introduced Angel Spawn, smaller less powerful, less dangerous spawn of the Angels, which shared physical characteristics, but were much smaller, and less central to the plot. While the main characters and the EVAs battled the Angles, the FanFic raced off following containment teams and secondary mecha pilots who were fighting the angel-spawn. As I recall, as the fanfic progressed, the contamination zone kept growing, as destroying the parent angel didn't wipe out it's spawn, and well into the fic series, the spawn of the first few angels remained constant problems.
I loved this idea.
Then in the Final Fantasy X game, the world was continually threatened by a massive soul/anima kaiju called Syn, and where it went, it would shed monsters from it's body that would spread death and destruction as it carried out it's mission of annihilation. These Synspawn functioned as minion fights, through boss fights, and some Summoners demonstrated their power by destroying these synspawn.
I loved this idea too.
And then rather obviously there were the Zerg. Hissing, gurgling, dripping with slime, and existing as a massive biocancer, the massive swarms radiated out from gigantic brains. The hydralisks and zerglings were all manifestations of the ever larger and larger creatures, up to the city sized Hivemind, and all came from slime crawling worms a few feet long.
There are a large number of kaiju who have strong design elements that would look fantastic spread out across a number of creatures dragonkin style. The Angels from NGE immediately come to mind, how large of a brood could aquatic angels like Gaghiel have supported, or Shamsel, who looked like a sea monsters? Likewise, Ramiel came to mind, with it's spawn being manifested crystalline entities, floating through their respective environments, being basically passive unless messed with, them responding with beam attacks. The classic kaiju come in too, especially the Toho/Godzilla gallery. Winged Godzilla spawn, tank like Ghidorah spawn, Mothra creating clouds of spores that grow into meter long voracious carnivorous caterpillars... The potential for horror is endless.