Shetland saddle fitting goes wrong through scale. The pony is small, broad, and short-backed, and the saddles offered for her are often scaled-down horse saddles, too long in the panel, too heavy, and shaped for a back with more wither and more length than she owns. The checks that matter are proportion checks: length against her short back, tree shape against her flat one, weight her small frame should carry in tack, and stability on a barrel that offers a saddle almost nothing to grip.
Length: the panel and the last rib
Find her last rib and mark, mentally or with a thumb, where it curves up towards the spine. Her saddle's bearing surface must end before that point, and on a Shetland that distance is short, shorter than many pony saddles assume. A panel that reaches onto the loin presses on unsupported back with every stride, and because the rider is light, the pony's protests, tail swishing, ears back at girthing, reluctance to go forward, get filed under naughtiness instead of discomfort. When you try a saddle, check its panel length on the pony before anything else, and be prepared to look at genuinely small saddles rather than the smallest of a horse range.
Tree and contact on a flat, wide back
Shetlands are broad and flat with minimal wither, so the tree needs to be wide and flat-shaped to sit down on the back with even panel contact. A peaked, narrow tree perches, pinches at the points, and guarantees rolling. Girthed, run a hand under the panels for even contact and check the channel clears her spine, which sits in a well-covered back where pressure hides. Clearance over the wither is usually easy on this breed; contact and stability are the real tests. Flocking in small saddles is often thin and tired, so check the panels are full and even rather than lumpy, since on a small bearing area a flocking lump is a large fraction of the contact surface.
Stability on a round barrel
The classic Shetland experience is a saddle, and sometimes a rider, migrating sideways on a turn. Test hers at walk and trot, on circles both ways, with a watcher behind. A fundamentally right saddle stays centred with normal girthing; one that travels needs its tree and panel shape questioned before accessories are blamed. Girth position deserves attention too: her girth groove sits close behind the elbows, and a girth that drags the saddle forward will pinch and rub in the armpit area and push the saddle onto the shoulder. A shaped girth that sits in the groove without hauling the saddle forward is a legitimate fine-tune. Check after the ride where the saddle actually finished, not where it started.
Weight, the rider, and girthing behaviour
Check the final fit with her real rider mounted, because a child's weight settles the saddle differently from an adult standing in one stirrup to peer at clearance, and the rider's position on that short back is part of the fit. Treat her girthing behaviour as evidence throughout: a Shetland who has begun biting at the girth, moving off at saddling, or humping when first ridden is frequently a pony reporting a fitting problem in the dialect available to her, and that report deserves a saddle check before a schooling response.
Record the pony-sized answer
The saddle that genuinely fits a Shetland, in size, tree, and panel length, is a find worth documenting before it is outgrown or replaced. KinsLe's Pet Passport keeps your Shetland pony's saddle size and fitting notes together with her records in one portable packet, so the next saddle is chosen from measurements known to suit her short, broad back.