It’s a phrase I’ve never, ever really thought about. Why do we say, “back in the saddle” or “swung up into the saddle?” We mount a horse, or get onto a horse, rarely fork a horse (old term, very rarely used today in most places). But saddles are always “into.”
You have to go back to the Middle Ages, and then to the Spanish saddles brought to the New World. Don’t think about a modern English style saddle, which is designed to be light and to allow the rider and horse the greatest sense of contact between them.* Think back to who owned horses and rode them and what most saddles were used for: either stabilizing cargo, or war.
The goal was to keep the rider seated no matter what, especially when you rode with straight, extended legs. The saddles had high pommels and cantles that secured the rider when he was hit from the front or behind. It also helps keep an injured or exhausted rider from sagging and falling backwards or forwards. Sideways is also a bit of a challenge, but you can fall off in any direction if you try. Or the horse helps you.
This is a replica jousting saddle. You get into it, not onto it. Creative Commons fair use. Original source: https://www.pinterest.com/pin/406520303850887415/
Here you see a version of the saddle in action, modified for the rider’s armor. http://www.thejoustinglife.com/2014/03/re-creating-medieval-and-renaissance_20.html It’s a great article about the how and why of re-creating a medieval saddle, and the results for horse and rider.
A 15th century saddle for show or parade. However, you can see the height of the pommel and cantle. This one is in the Met Museum. https://dariocaballeros.blogspot.com/2010/08/xv-century-medieval-saddles-from-met.html
These are the type of saddles most writers talked about, unless they described a specific sort of other saddle – pillion pad, or pack saddle, or box saddle, or side-saddle, or . . . These were the sort brought to the Americas by the Spanish, and used on war horses and mountain horses. Below is a Spanish colonial saddle from the late 1700s-early 1800s. It should look familiar.
To this day, charro saddles and others have higher, more snug pommels (or swell) and cantles than do some other western saddles. Because of this Spanish tradition, it is common in American English to say that you get into the saddle, even if you are riding English style.
Some modern dressage saddles retain the older medieval features if you are doing the Airs Above the Ground (Austria and Portugal, France are about the last places to learn this tradition.) I have a barrel-racing saddle that I love. It has a relatively snug seat, as well as suede on the outside, so the rider fits more snugly for making tight turns and rapid accelerations/decelerations. I got to sit in a replica medieval saddle once, and it really helps lock in your posture and where you put your weight.
And so, Roy Rogers and Gene Autry sang about “Back in the saddle again,” because that’s what vaqueros did, and what modern cowboys do. And reenactors, and advanced dressage, and . . .
*Bareback’s no fun for extended periods, and not stable. For either horse or rider, especially once you shift into the trot or canter.