I'm looking for a specific fic that I read a couple years ago. It was set during the CA:TFA, and had Steve/Peggy/Bucky as the endgame ship but was a WiP. Bucky was a merman, and for some reason he needed to transform shortly after being rescued from Azzano. He and Steve went to a river, and Peggy saw him with a tail and found out.
Steve and Peggy had been working towards a relationship, and then Bucky started flirting with her. She turned him down and said she wasn't some toy for him and Steve to share. Later Peggy came across Steve's sketchbook. He'd drawn himself as an ape with merman-Bucky and angel-Peggy (or possibly some other mythical creature) sitting on his shoulders. Her first thought was 'wow this is careless of Steve to leave his bff's greatest secret so easily stumbled upon, rude' and her second thought was 'Oh, the ape in the middle, I totally misunderstood that earlier, Steve being in love with two people is way better'
I think that's where it left off, and even if no more has been posted since I'd love to read it again if anyone can find the link!