Mermaid Cocoon Newborn Photo Prop
This fits newborns under the arms, and older babies at the waist.
The tail is sewn onto the bottom.
The stitch pattern is not as difficult as it looks. It involves slipping stitches with the yarn in front, and then knitting the loop at the proper place on a subsequent round.
You may choose to knit it back-and-forth or in-the-round.
The stitch pattern is quilted lattice. There are videos on YouTube that show how.