Many shaped saddle pads are made of real wool or synthetic fleece.
A shaped (or "numnah") saddle pad is saddle shaped, and is designed to be used alone, or with a square saddle pad. A shaped / numnah pad should only be slightly visible below the saddle.
Sheep wool fleece pads provide more cushioning than synthetic fleece pads. However, wool fleece becomes matted and compacted over time, and requires more maintenance to dry from horse sweat. Synthetic fleece pads are not as cushioned as sheep wool fleece pads. However, synthetic fleece lasts longer over time, is easier to dry from horse sweat, and doesn't pull on horse hairs.