Build your stable’s Prestige and return to the trail selection to check the mountain route. The full-game description confirms mountain trails, and the developer has explained that Prestige controls which trail rides become available.
Advance the Stable First
Choose a practical way to gain Prestige: improve buildings, care for boarders, or win competitions your horses are prepared for.
Check the Mountain Option After Progress
After your Prestige rises, inspect the available routes. If the mountain is still locked, follow any requirement your version shows and keep progressing.
Choose a Horse for the Outing
Look at the horse’s condition and workload before leaving. You can use a currently available outing while your stable works toward the mountain route.
See the official Horses of Hoofprint Bay Steam page for game information.
FAQ
Does the full game have mountain trails?
Yes. Steam lists riding through the mountains among the trail options.
Does a building upgrade help?
It can raise Prestige, which the developer links to trail availability.
What exact Prestige do I need?
The cited developer material does not confirm a fixed mountain threshold. Use your game’s trail requirements.