I’m sharing the pattern I used to make the doll into a mermaid, you still need to get the pattern, this is only for the tail, fins and ruffle.

Tail
sc 6 into a magic circle
sc twice into each stitch
(2sc, sc1) repeat around
1sc each stitch for 3 rounds
(2sc, sc2) repeat around
1sc each stitch for 3 rounds
(2sc, sc3) repeat around
1sc each stitch for 5 rounds
(2sc, sc4) repeat around
1sc each stitch for 5 rounds
(2sc, sc5) repeat around
1sc each stitch 5 rows
(2sc, sc6) repeat around

-switch to body color and continue with top of body starting at round 38 (or where you want, it depends how long you want her body

Ruffle
Find the front center at the top of the tail, join your yarn 1 stitch over.
2sc in each front loop around until you reach the front again, leave 2 or 3 stitches empty, ch 1 & turn
2sc in each stitch. Fasten off and weave in yarn.

Fins (make 2)
chain 11
sc into 2nd chain, sc in each ch until the first, 3sc in the first ch, rotate piece so you are crocheting into the back of the chain, sc across, ch1 & turn
Now work in backloops for each stitch
(slip stitch in first 3sc, sc to the center of the 3sc from previous row, 3sc in center sc, sc across leaving 3 stitches, ch1 & turn) repeat for 4 rows

This doll has been in my head for such a long time! Time to crochet her!