I can see why he was called "Lucky". I'm afraid this is a natural reaction of stallions in the wild to another stallions progeny. Thankfully the foal was not killed or maimed worse and although he has a scar it is a long way from his heart and shouldn't affect his future performance life.
I really enjoy this blog as I used to work at Hagyards several years ago and it brings back great memories. Can't wait for the book.