These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Genius Bonus: The three Inhumanoids represent all three of the scientific "kingdoms", and these three kingdoms arguably help define the Inhumanoids' respective roles.
Mineral: Metlar. Since the planet itself is made of various mineral substances, it makes sense that he's the leader.
Animal: D'Compose. Quite a few animal species are at least somewhat intelligent, so it makes sense that he's intelligent as well, making him Metlar's Dragon.
Vegetable: Tendril. No plant species are known to have any intelligence as we understand it. In fact, the term "vegetable" is often used to describe a person whose higher brain functions have stopped. It wouldn't make sense for Tendril not to be a Brute.
Let's look at the basic concept; there are Eldritch Abominations at the center of the Earth... and they want to wipe. Us. Out. They can appear anywhere on the planet, wreak havoc to their heart's content, and then vanish before we can react. And did we mention they're all the size of kaiju?
One such monster, Metlar, is a gigantic demon of living metal & rock who can produce globs of magma/molten iron from his mouth and use them as projectiles, he can animate any ordinary stone or metal statue as a loyal slave-warrior, and did we mention he's implied an interest in what humans taste like?
Another such monster, Tendril, is a hulking mass of vines, a mountain-sized array of Combat Tentacles that can grow clones of himself within seconds of being shot or sliced apart.
The third monster, D'Compose, is a sort of decaying zombie dinosaur who can both reanimate the dead as his mindless slaves and, with a touch, cause a living human being to transform into a decaying, gargantuan, monstrous version of themselves. He gets to show this off in the second episode. The results are... not pretty.
Episode 9 introduces the crocodilian-looking Gagoyle, a baby monstrosity that hatches from a man-sized egg and then starts ruthlessly devouring its siblings, eggs and hatched ones alike. By the next episode, it's fully grown and is showing off a see-through stomach. What's worse is that this creature is explicitly a mindless eating machine; it exists only to devour everything in sight.