Both ones I know of is Kagome as a mermaid.
My Mermaid - TamashaToko
AU Things turn for the worse when Kagome is captured and turned into a mermaid and the only one who can save her is the arrogant bussiness man Sesshoumaru. SessKag
Rated T on ff.net so can't be too bad. Written in 2005 by me so probably not the best written thing on the block.
the mermaid - kay wiz
- THIS STORY WOULD BE RATED MA ON OUR SITE FOR LANGUAGE, NO YIMs!
Canon, not AU. Kagome secretly transforms into a mermaid every night. Torn between the mermaid and the miko, Sesshoumaru discovers he isn't as immune to love as he claims to be. But when feelings get complicated, how will Kagome tell him who she really is?
Also rated T. I remember really liking it.
Sadly both fics are incomplete