Hi Ralph,
Air inflation will always follow the second law of thermodynamics, entropy can only increase over time in a closed system. That is the physics behind making 2D shapes into 3D shapes with air inflation. Pretty worthless practically.
But for your horse, let’s talk about what you can do. Obviously, you can make a leg with a bend 2D and have the top wider than the ankle to be more realistic. The trick will be in attaching the “cylinder” leg to the “cylinder” torso. I hope this makes sense, but in some case you could be welding with 3 pieces of material in the equipment when you only want to weld 2 pieces of material. It is an easy trick to use a thin piece of teflon sheet between the 2 pieces of material you don’t want to bond. But you need to make sure you don’t “trap” the teflon in the finished good. Your manufacturing steps need planning.
Ways to make shapes not always “cylinders” pretty much comes down to 2 methods. Spot welding between the seams. This will create a tufting effect. The other is to use flat gussets between your two primary sheets. For example, imagine how they wrap aluminum (your primary sheet(s)) around the the multiple frame elements (gussets) that make the wing profile. The only downside is you will see the weld marks of the gussets on your primary sheets.
Like I mentioned in the other thread, get a roller sealer, teflon film and PE film from McMaster and experiment. It’s fun.