Selkath

They are capable of breathing underwater for massive periods of time, and are the third race who are capable of directly manupliating magic, with them being more effective with water magic. The other races capable of magic are ponies(Unicorns, not pegasi or Earth ponies) and humans.

Discovery
The Selkath hail from the planet of Manaan and were first contacted by the Triarian Collective, who invaded their homeworld and their only colony of Morn to get to an artifact left behind Eons ago that they needed to defeat the Darkness. They were enslaved for a a short period, which lead to the Battle of Morn. After the war they were independant for many years, and did not move into space much again, before joining the Equestrian Empire.

Time under the Equestrian Empire and Splinter Group
Under the Equestrian Empire, the Selkath grew in power. They were given magic, and naturally are better with water magic than they are with any other branch. Eventually, three worlds on the edge of Triarian Space split off from the Equestrian Empire and joined the Triarian Collective. The majority of the Selkath are under the Equestrian Empire, but there are three worlds under the Triarian Collective. Unlike most other Collective races, there has not yet been an augmented version. This is presumably because the Triarii are attempting to find a way to do it without losing the odd bonus of magic, as the Selkath are the only species within the Triarian Collective able to use magic