Sometimes, the lead stallion can have a co-stallion who is either their brother or close friend. They may scuffle but they generally share the mares and protect them together.
Of course, there's the alternate path. A human finds you, and you make a deal. They feed you and protect you, and you carry them. There are numerous side quests that branch from this, each with their own interesting story.
Actually, the young mares (fillies) are also expelled from the herd around the time of their first heat. While the colts will form batchelor bands, the fillies will be quickly picked up by roving stallions and added to their herds. This prevents inbreeding. Wild mustangs will not normally breed with their offspring unless forced to do so.
The thing about stallions: they usually do not guide the herd to food or water sources. That's actually leading mare's job. Leader stallion just protects his group from various dangers.