I have a boxer with a small circle on the back of her ear. It looks like a donut with hair in the middle. It's the only one on her body. I can email a picture if it helps. Is it ring worm or mange? Or just a bug bite....we live in a wooded area in MI...if it helps.
I would need to examine your pet and run some tests to determine the difference. It is hard to tell from looking at it alone. Your pet should be seen by a vet b/c 2 of the 3 can be contagious. Vetcare