Grape-sized tumor under the skin

Hello laurie,
This could be a chronic inflammatory reaction, say to a deep bug bite that is fibrosing or a neoplasia of unknown carcinogenicity. Most subq bumps of this type are not metastatic cancer but without proof who can say? If you and your veterinarian are worried this may be cancer I see two choices: a needle biopsy or removal. If the horse is very compliant and this can be done standing, I would opt for removal and then depending on its appearance maybe histopath.
DrO