The stallion ad might say dun but that horse is 100% buckskin. He may have nd1 which is not dun but can give a horse primitive markings - which will be where the dorsal comes from. It is very clearly an nd1 or countershading dorsal not a dun dorsal because it stops at the tailhead rather than continuing through the tail.
The only mountain and moorland (Great British native) breed that has dun is the Highland Pony. Connemaras do not have dun. However they all incorrectly
call buckskin "dun".
This is a dun dorsal stripe:
The above horse is a grulla (black dun) but the image clearly displays what I'm talking about in regard to the dorsal stripe continuing through the tail as compared to the OP horse, where the stripe ends at the tailhead. Additionally the horse in the OP has no other clearly visible primitive markings such as shoulder/leg barring, dark bars on the ears, or a mask on the face. He is a sooty buckskin. As sooty is progressive he will gradually darken.
In comparison, a bay-based dun Highland Pony:
Note the darker face and the barring on the legs. This photo isn't ideal to see the dorsal continuing through the tail.
A "dun" Connemara (actually sooty buckskin):
Note the lighter/same colour face and the lack of leg barring.