Things to unlock in higher levels
Level 3-4: 7 new events (+ new event type), 1 new weather condition (+weather related events)
Level 5: 5 new events; level 5 treasure-event, 3 new weather conditions, 1st Story unlocked
Level 6: 4 new events, 3 new weather conditions
Level 7: level 7 treasure-event, 2 new weather conditions, 1 new Story
Level 8-9: 10 new events (new NPC rival), 4 new weather conditions
Level 10: 6 new events (new NPC rival)
Level 12: Protect up to 9 horses, 1 new weather condition, (Starhorse)
Level 15: Unlock leadmare function
Level 20: Special surprise horse in events
More things to unlock in higher levels will follow!
Small Herds: Secret passage (not implemented yet), general help/protection for smaller herds
Big Herds: Horse wants to join your herd (35+), Injured herdmember (50+), Thunderstorm (60+), Bigger Fire (100+)
Herdsize related leadstallion and stories will follow!