Every other fitting conversation can start from a breed shape. A Mustang's cannot. Mustangs come from varied origins and it shows: one is a compact, mutton-withered horse of Spanish type, the next a rangier animal with proper withers and a longer back, and both answer to the same name. So the checks for a Mustang are the universal checks, applied with unusual discipline because there is no shortcut of expectation: measure this horse's width, this horse's back length, this horse's curve, and expect the answers to change as a newly started Mustang builds muscle he never carried in the wild or in a holding pen.
Width: measure the shoulders in front of you
Stand him square on level ground and assess the angle and spread of his shoulders and back where the tree will sit. Many Mustangs are medium to medium-wide, but assume nothing: fit the tree to the horse, checking that the seat sits level ungirthed, the points do not dig into the shoulder, and the saddle neither perches nor drops. A fitter's eye is worth more on this breed than on any other, precisely because pattern-matching is unavailable.
Contact: bridging and rocking on an unknown curve
Girth up and run a hand beneath the panels along their whole length. You are feeling for the two classic faults: bridging, where contact exists at front and back but floats in the middle, and rocking, where the saddle pivots because its curve and the back's curve disagree. Mustang backs are often short and strong, sometimes slightly roached, sometimes dipped in a horse still building topline, and each shape argues for a different panel. Check the sweat pattern after work once he is riding: an even pattern supports the fit, while dry patches at the front and rear with sweat between, or the reverse, maps the pressure for you.
Length and withers: the common Mustang combination
Short backs recur in this breed, so measure behind-shoulder to last rib and keep the panels inside it, resisting saddles that look proportionate but bear on the loin. Withers are the other variable: many Mustangs have low, wide, mutton withers, which give a saddle little to key into and let it slip forward and sideways, while others have perfectly defined withers that demand normal clearance checks. Judge the horse you have. For a mutton-withered Mustang, clearance over the wither is usually generous but stability becomes the battle; for a higher-withered one, verify the mounted clearance and channel daylight as you would on any horse, and recheck once the saddle settles under way.
Stability in motion, and a horse still changing
Test at walk, trot, and canter if he is there, on turns and slopes, with a watcher behind. Mutton withers plus a round barrel is the classic recipe for a saddle that travels, and the fix is correct tree shape and girthing arrangement first, grippy pads as fine-tuning, never brute girth tightness. Remember too that a Mustang in his first seasons under saddle is a moving target: topline develops, shoulders broaden, and the fit that was right at backing may pinch within a season. Schedule re-checks as his work increases, and treat new behaviours under saddle, reluctance to go forward, tail swishing at girthing, dipping from the mounting block, as fitting data worth investigating before training conclusions are drawn.
His measurements, kept as his own pattern
Since no breed standard will ever predict this horse, his own recorded measurements become his template. KinsLe's Pet Passport keeps your Mustang's tree width, panel notes, and fitting history together with his records in one packet, so as his shape develops you can see exactly what fitted him at each stage.