A Horse&Rider answers:
The hair in and around your horse’s ear traps dirt and dust, preventing them from entering his ear canal. A certain amount of wax is normal and can be trimmed off to keep him looking smart.
However, it’s unusual for one ear to have more wax than the other, so it’s worth asking your vet to take a look – especially if your horse is showing other signs of irritation, such as headshaking or reluctance to have his bridle put on. It’s a good idea to ask your vet to look down his ear with an otoscope. However, lots of horses don’t much like this process, so he might need sedating.