Hagure Ningyo no Diana

A certain country long ago. A mermaid was brought to the castle. The rare mermaid with transformed legs, was supposed to be presented to the king of a neighboring country... But the prince who saw the mermaid said, "Would you like to be my princess?" A communal life that starts from there! How will the love of these two people go...!?

(Source: Shogakukan, translated)