Fleece bottom saddle pads with a straight / square back are a traditional western saddle pad style.
The square (or "straight" or "rectangular") saddle pad is for horses which have straight, or round, mutton-withered backs.
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.