It’s probably just a rub, but it could also be a skin condition. Are there bumps? Is it pink/white/discoloured? Is it scaly, crusty, or hard? Where specifically is it on your pony? If there’s nothing abnormal about it, other than it being a bald spot, and as long as your pony isn’t biting at it/excessively itching/ showing any signs of pain when you touch it it’s probably nothing major, but you should have someone look it over if you’re unsure.