The most recent origami from Lang’s book Origami Design Secrets: Mathematical Methods for an Ancient Art that I have folded is the koi fish. It was fun to make. It looks complicated, but it isn’t. Lang does show how to make scales, which you can implement on the koi. However, this makes it much more complex and time-consuming, so I started with just the non-scaled koi.
I might try to make some scales on a plain piece of paper, just to learn how to do them. If they are easy enough, then I might try folding a new koi with scales.