I have a 7 year old Canadian mare that I have had since she was 3 month old. She has lives all year around with an old gelding and 2 mini donkeys for a very long time. When a new gelding came she was extremely aggressive towards him. She was biting and kicking. Also, I went for a trail ride with a friend of mine and my mare was alright towards my friends horse during the ride. Later, the two horses were grazing in-hand when their heads got a little closer (about 4 feet-ish). My mare pinned her ears back , swung around, and kicked my friends mare.
What do you think this is about? How do I fix it? Should I punish her for being aggressive or is it just natural?