I just saw some neat tree protectors on the live feed from Badminton.
It looks like they set three posts in a triangle around the tree fairly close in, added two or three rails across all the way around, and then either attached or wove very tall thin poles (saplings trimmed from elsewhere?) through the rails.
Horses wouldn’t be able to get near the trunk to strip the bark off, but they can still stand under the tree.
What other things have you done to protect trees in your paddocks or pastures?