123D Sculpt
Traditional sculpting is a difficult and messy business, but 123D Sculpt by Autodesk simplifies things a lot. Designed specifically for the iPad, it comes with a tutorial that will get you up to speed. You can create human heads, dinosaurs, birds and other complex shapes.
There are tools for smoothening, pulling and pushing a model. Use your fingers to twist, pinch or swipe. These sculpting tools can be selected directly. You can sculpt the mesh or the model. Some creases appear on the reverse side. However, this really isn’t a problem because there is a symmetry mode.