Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
This area is a common one for Cuterebra larvae. This si a fly larvea that has been laid in an abberant host so the larvae die and cause this lump. Try to squeeze these lumps and see if there is something that looks like an air hole....once the larvae die, this will become an abscess which may need to be lanced by your vet if you cannot get the organism squeezed out. This is probably why these are becoming larger.
If this is not a larvea., they may be scar tissue that will resolve on its own. The fact that 2 are getting larger may indicate an abscess that will require treatment.
So, to answer your question, I would be concerned about these....these are not serious but can become painful to your mare if left untreated.
Does that make sense to you?
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