The Aquanorns were drawn by Tomtschek with a genome by Alien. They are amphibious, and breathe a lot better underwater then they do on land. They go from having legs and a developing fish tail, to only having the fish tail. Once this fin has grown, they can swim. Given this similarity, it's possible they have at least a spiritual relation to MerNorns.
The New Aquanorns are a genetic variation on the original Aquanorn breed. They have CFE and modified CFF edits, the Gizmo Norn's biochemistry, they have the Swim Bladder edits, and they use Grendel Man's swimming agent. They are available at The Realm. They do not include the sprites or body data of the original Aquanorn, only the new genome.