A species of highly intelligent aquatic beings, the Hydraequa lived on their home planet of Atlantis heedless of the conquests and happenings of the galaxy outside their seas. This all changed when the human surveyor teams came across the world. The Hydraequa saw their planet of Atlantis as home. The humans really just saw is as prime real estate. Businesses sprang up and developed the land, building massive five star hotels and restaurants underneath the sea, turning the entire place into the largest tourist trap in the galaxy. They rounded up the Hydraequa and quite a few of the other seabourne fauna on Atlantis to use as mounts. This prompted war between the humans and the Hydraequa.
They unleashed their formidable military industrial complex, striking back at the humans and prompting a vicious attritional war against them and the Empire. Unfortunately, as it came down to attrition, there was no way for the Hydraequa to endure. The Empire was too vast an enemy and soon enough they lost, resulting in their enslavement and collapse of their civilization. The few who avoided this fate gathered what technology they could and fled to the stars, eventually coming across the Mi-Go. When they discovered that they and several other species were organizing a total invasion of human space, they joined without hesitance, desperate to take vengeance for their people.