Aquatic Elf
Aquatic Elves, as their name suggests, are a branch of the elf family tree that have taken on an amphibious lifestyle, leading to them being sort of an interstitial state between elves and merfolk; gills and webbed digits allow them to live and move quickly under the water, but a bipedal bodily configuration lets them leave the water and adventure alongside other humanoids.
Though they suffer from the same problem as any other aquatic race in that they are generally unpopular, aquatic elves have appeared throughout the history and settings of Dungeons & Dragons, having appeared in The Sea Peoples for BECMI, the Complete Book of Elves for AD&D, the Monster Manual 1, Stormwrack, Races of Faerun and Races of Ansalon for 3e, and Inner Sea Races for Pathfinder.
The Dragonlance setting has the most interesting aquatic elves, being divided into two species - the Dargonestic ("Deep Elves", who can shapeshift into dolphins) and the Dimernesti ("Shoal Elves", who can shapeshift into otters).