Good Morning Samantha and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
If this lesion is near the umbilicus , this is most likely a plaque left over from a fly bite abscess. This is common in many horses because this is the area that flies go to and the irritation can cause an abscess or such severe irritation that this plaque is formed. It is a good sign that it does not bother jack because this means that this area is healing. the best treatment for this is a cortisone creme applied to this area every 6 hrs and then applt a fly spray or a product called SWAT that is found at the farm supply and keeps flies away.
If this does not help, Samantha, have your vet examine this area since tumors or infections can cause this as well.
Does that make sense to you, Samantha?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, Dr. Loretta