User blog comment:Jamintheinfinite/Subnautica Creature Evolution/@comment-28450065-20181006163448/@comment-28450065-20181011184129

Because the Oculus entry tells us that they originated from Peepers that were trapped in a cave. That would have forced anatomical changes but since they still came from the common Peeper they're very close relatives despite not looking that similar the arctic Peeper probably split longer ago but did not have the same pressure put on it to change like the Oculus did.

Most of the idea of them being closer comes from the shared fin design.