Some saddle pads have a raised contour at the withers, and are made of neoprene to provide a non-stick surface on the horse's back.
Contoured (or "built-up", "wither-relief", "swayback" or "orthosport") saddle pads are raised in the front to lift the saddle off of the withers. Built-up saddle pads are designed to fit horses which have very high (shark fin) withers. Swayback saddle pads are designed for horses which have swaybacks or very prominent withers. Swayback pads have an elevated area over the withers, and also provide extra padding in the middle of the pad. Orthosport pads have a breathable core, and extra padding behind the shoulders and along the spine, to relieve pressure from the withers and kidney areas.
Neoprene is a synthetic, closed cell rubber. Neoprene saddle pads are good at distributing pressure, and at preventing sliping and clinging to the horse's back. Disadvanteges of neoprene are that it does not wick moisture, and can trap heat.