When Jim sees the wild horses & gets thrown off the colt you can see a man on a horse chasing (herding) the 'wild' horses.
Also, after this scene when Kirk Douglas sees the colt has been ridden there is a girth mark on the horse, when Jim jumped on the horse he was bare back.
The saddle and girth marks on the colt were from being worked in the corral prior to the bareback scene. I don't feel like that was a mistake. Surely, though, the man chasing the brumbies was indeed out of place!
If you watch the scene Jim was brushing the colt when the brombies came running through. There was clearly no girth mark around the horses belly, and if you recall both scenes showed the "on" or left side of the horse. If there had been a girth mark you would have been able to see it