From Creatures Wikia
The Fish Ettins are a genetic ettin breed by Grendel Man. They can swim with the Aquanorn swimmeragent, and they use the sprites of Guppy Ettins. They have gills, which make them unable to breathe out of water. Therefore it is best to keep them in a water area that they cannot leave. They will occasionally eat a critter.
The Fish Ettins are available from Grendel Man's section in the Norn Adoption Center.