User with this ability either is or can transform into a fish people, category of (usually) intelligent, underwater fantasy species. While they're humanoids (one head, two arms, two legs), they don't look human; their bodies, especially the faces, have a fish-like/reptilian appearance. Their disposition/attitude towards others (especially surface folk) and ability to survive on land vary depending on the character and the series.
While Fish People are most often a pre-existing race, sometimes they are former humans (or fish) who are the result of mutation. This origin seems especially prevalent in Western Animation.