These are the pants for a Little Mermaid costume for a cute little girl who's turning four soon:
It's not blurry edging on the 'fins', there's a layer of netting-type fabric in front. The fins pretty much stand up on their own. These were pretty simple really, but I'm still proud of how they turned out. I took a basic pants pattern, then drew a fin freehand, cut 4 in satin and 4 in netting, assembled the fins, and just sewed the fins in as I was doing the outer leg seam of the pants.