if your mare has a bump filled with fluid, it can be an abscess.
You can put green clay on it or betadine during a week and see if it goes away, if not call a vet to make sure this is not something more serious.
If it's infected, you will see liquid going out of the bump, in this case call the vet immediately, he will give you advise on how to treat or even treat it himself.
Knowing what you said, i don't think that it's a tick because it wouldn't have fluid, it will be red and you will see a bite mark but you can look if you see one on him by precaution (if there is one, be careful to remove the head).
I hope i could help you, don"t hesitate to contact a vet just to be sure :)