Dremeton

The Dremeton are the only intelligent species native to Elithia. But it is home to the Dremeton, Aegi and Craton.

Aquatic creatures with long, slender bodies and large fins on the ends of their tails, along with a single large dorsal fin. They have evolved two arms with three-fingered hands, and a crystal in their chests is used to measure water pressure and adapt accordingly

The Dremeton are an aquatic species, so they use exosuit leg attachments to move around on land. Most of the Alliance species are land-dwelling creatures, so the Dremeton often need those legs on diplomatic missions (or just for tourism). They also need special breathing devices and a coating that keeps their skin wet, since they don't last too long outside of the water.

Their nation is called, the Dremeton Union of Cities, they're ruled by a High Council.

The Dremeton have largely abandoned religion, like the Aegi did. Dremeton are supposed to have a rich mythology though, after their lore overhaul.

Inter-race interactions
Dremeton were the first race to encounter the Notix.

Dremeton were still primitive when they first encountered met the Aegi, there wasn't much communication as the nations didn't understand each other. They initially attacked the alien species and the Aegi retaliated.

Back then, they were still quite early in their evolution as an intelligent species, using stone and metal weapons with only basic settlements underwater.

Some Aegi (who later founded Creon City) wanted to try and establish some form of diplomacy between the two species, whilst others (those who founded Lonro) simply wanted to claim a territory further from the sea to prevent further (hostile) interactions.

Yeah, the Aegi/humans would win almost every conflict, though there weren't all that many and they were all pretty small in scale. The Dremeton are aquatic creatures and weren't able to do much on land, so conflicts were mostly on the shores and on the waters. Human fishermen getting attacked by Dremeton from below and such.

In the end, the inhabitants of Creon succeeded, and the name "Aegi" was actually given to the humans by the Dremeton.

The Alliance started with the first interactions between the Aegi (who were still called humans back then) and the Dremeton (who are native to Elithia). At first, the two species had some conflicts, and the matter of how to deal with the Dremeton split the human faction in two. The cities of Creon and Lonro were built by the two human sides, and that original split is why the Union grew into a federation and not a single state. When the inhabitants of Creon finally started making progress on cooperating with the Dremeton, Lonro eventually joined with Creon again to form the Aeginian Federal Union. The Aegi and Dremeton later signed a common set of laws, the Elithian Code, which the Craton ended up signing too.

When the Craton eventually assaulted Aeginian and Dremetonian transport - breaking the code - the Aegi and Dremeton started working together even more closely, and thus the Elithian Alliance was born.

The Alliance have a grudge against the Kharakorum Domain - the Craton have a long history of war with the Aegi and the Dremeton, and while the latest generations of Craton have been more stable than the older generations, there's still a lot of anger.

Architecture
Dremeton architecture will use rounded, flowing shapes, often staying low to the ocean floor with only occasional 'highrise' construction. They will often build their cities in natural crevices, depressions and underwater ravines.

Language
The Dremeton do use a lot of clicks in their communication, but also have sounds that sound a lot like a human voice. They have various forms of communication through sound: they spend most of their time underwater, but rise to the surface often too and can live in very shallow water as well, so they've developed the ability to make sounds fit for both underwater and above water.
 * Gar-Aeroix is Dremetonian for 'Great Planetary System'.
 * In the Dremeton language, the sound they used for "friend" sounded like "Aegi", and the name stuck (the humans saw it as a great way to start over on their new planet).

Trivia

 * Dremeton are unlike any creature found on earth, and as such they can't be qualified by Earth categories. There's plenty of similarities too, though, so while none of the categories are a perfect fit, some fit better than others.
 * The Dremeton are due for a large overhaul, and a lot of things about their exact physiology may change. They'll most likely have live birth, though that isn't set in stone yet. That may actually make mammals a good fit for them too, but their respiratory system won't really fit that description since they are a true aquatic species which needs a lot of help to survive above water.
 * The Dremeton, while they were created by Aegonian, were also under Veoline's control for most of the old RP, which is one of the reasons the former hasn't done much with them. One the one hand, Aegonian don't want to undo all of Veoline's work on the Dremeton, but one the other, he wants all canon lore to be his own for IP reasons and potential future stories.
 * They were originally on that list of species that people could choose from in the old RP. Aegonian's trying to reintroduce the Dremeton to the canon storyverse (and Starbound!) again.