I believe that it can be genetic. There are some horses that just have floppy ears.
Nerve damage would make a lot of sense actually :)
No Alexia, there's no muscle atrophy.... She sees vet regularly and has not had any ear mites, weird fungus infections. Her ear started gradually drooping maybe 3-4 years ago. It wasn't all the time, or bad, to start with. Hardly noticeable at first, the vet doesn't see what's wrong, either!
Is there any muscle atrophy around the base of the ear? Probably muscle / nerve damage!
The only horse I've ever seen do that was much more dramatic and she'd hit her head very hard and had a stroke. It might be nerve damage at the base of the ear.
Sound like she might have done a bit of nerve damage once when she hit her head. I can't really think of another reason for it!