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.