革 on the left tags 鞍 as a leather-craft word; the indexing radical here marks the saddle as a tooled hide product. Same radical groups it with 鞭 (whip), 靴 (boot), and other hide-and-strap goods.
安 supplies the reading ān with no tone shift, and offers a faint semantic hint: a saddle is what lets a rider settle peacefully on a horse. Same phonetic appears in 按 (press) and 桉 (eucalyptus).
