There is miniature Doll body with human legs. You can try undine tail and hair on as separate parts.
You can make a mermaid with different colors of hair and tail. Doll’s size is 3’’ (7 cm).

If you know how to use a magic circle, single crochet and how to increase and decrease, you can do it!
Materials you will need cotton thread (100% Mercerized Cotton Yarn Thread) beige, green and yellow colors, and a crochet hook (1 mm - 10USA - 4UK), also you will need fiberfill for stuffing and needle and scissors.