[QUOTE=morganpony86;7099639]
I think these are different than what the OP is describing. Granulomas occur under the saddle, often associated with uneven saddle pressure. They’re hard, sub-q bumps.[/QUOTE]
From what I understand, saddle pressure is not always the cause. They can occur on other areas as well. My horse has several under the saddle and a couple on his flank. This picture is the closest I could find to what my horse has: http://www.vetstream.com/equis/Content/Disease/dis00950. They started out as bumps (probably insect bites), shrunk, and have ended up white or grayish, very small lesions. A better description:
In horses, the disease has been termed equine eosinophilic granuloma with collagen degeneration,nodular necrobiosis of collagen, and collagenolytic granuloma. The lesions are nodular, nonulcerative, and nonpruritic. They often are found in the saddle, central truncal, and lateral cervical areas and may have a gray-white central core. Older lesions may become mineralized. Both insect bites and trauma have been suggested as etiologies, although the occasional onset during winter in cold climates and in noncontact saddle or tack areas suggests multifactorial causes. Histology reveals multifocal areas of collagen degeneration surrounded by granulomatous inflammation containing eosinophils. Thus, histologically, this lesion is similar to eosinophilic granuloma of cats and dogs.
In horses, solitary lesions may be treated with systemic antibiotics, surgical excision, or sublesional corticosteroid injections. Mineralized lesions require excision. Triamcinolone acetonide (3-5 mg/lesion) or methylprednisolone acetate (5-10 mg/lesion) is effective. No more than 20 mg triamcinolone acetonide should be administered sublesionally because of the potential to induce laminitis. Horses with multiple lesions may be treated with oral prednisone or prednisolone at 1.1 mg/kg, sid, for 2-3 wk. In horses with recurrent lesions, intradermal allergy testing, particularly with insect antigens, is recommended. Hyposensitization and insect control can be palliative in some cases.
Impossible to say if this is what OP’s horse has as “lumps” can mean many things. 