Believe it or not, a little bit of sour kraut juice works really well on ears and legs. Just dab a little on, and don't put too much. This only needs to be done once a week until the fungus goes away. If the fungus is oozing or super irritated, talk to a vet immediately
It is very common, don't worry. You can make the funguses dry so they will fall easily. I know special product exist. You can also apply a cream but the less you touch your horse's ears, the better it is.