Yes- the Melodies, from the 8th moon of Yavin, are a species that look exactly like merfolk as adults. As children they have legs and live on land, though. https://starwars.fandom.com/wiki/Melodie
Wait so I'm reading the article on Sannah...was she in human form all the way from Lyric's World to TUF? When she becomes the adult form, she can't really do any more Jedi stuff, right? Unless I dunno, she uses the Force to levitate around or something...
The "form changing ceremony" takes weeks and involves algae native to Yavin 8. It usually occurs when a Melodie is close to 20. If the Vong "invaded and destroyed all life" as is stated in Lyric's article, it may now be impossible for Sannah to undergo the ceremony. Given Sannah's listed birth date in 13 ABY, she'd be 16 in 29 ABY during The Unifying Force.
Huh, so the ceremony is optional? Kinda seems like legs are more useful in general than a caudal fin...