One usually thinks of the western saddle as an American style of horse riding, but in fact it's origins can be traced back to the Moorish horsemen and warriors of the Dark Ages. When the Moors invaded Spain in the 700's, they brought with them their unique style of horseback riding and tack. Their saddles were designed for battle with longer stirrups to accommodate their armor and high cantles which provided them security in the saddle and protection from their enemies. So it was mostly Knights and crusaders that used these saddles.
The Spanish adapted the Moors saddle for their use in to what became known as the Spanish War Saddle. This was the same saddle they brought with them to the New World. As they changed from needing military gear to fight an uprising to the needs of colonial expansion, the military saddle was again transformed in to a stock saddle to better suit their needs. This was known as the Spanish Stock Saddle. It was designed as a tool for the working cowboy and evolved along with the expansion into the American West, from everything to fighting Indians to helping settlers round up their cattle.
Over time the stock saddle was influenced by geography and the culture using it. Very distinct styles developed reflecting differences in climate, terrain, culture and stock working styles. The harsh climate and punishing brush of the mesquite of the southwest resulted in saddles built to protect the rider, thus making it big, bulky, heavy and plain. California, on the other hand, had a mild climate with lush land where the vaqueros had much more leisure time. Their saddles were not only smaller than the Texans, but evolved into highly decorative pieces that are still used in show rings today with their conchos and elaborate designes of tooling.
From the 1700's through the 1950's, the western saddle continued to grow and evolve, with many new features to support cattle work and improved construction methods extending the strength, durability and comfort of the piece.
Today, the working cowboy isn't the only focus on saddle making. They are now designed for a wide variety of uses and riders such as trail or pleasure, endurance, rodeo contestant, team roper, barrel racer, reiner, cutter, and of course, working cowboy. Western saddles are now all over the world, but regardless of the style or origins, all of these saddles have an ancestry they share from the western stock saddle.