A fourteen-year-old Arabian stallion was presented with general weakness and hematochezia. Clinical examination showed tachycardia but the rest of the physical parameters were within normal limits. Transcutaneous abdominal ultrasonography showed distended small intestinal loops with normal motility but increased mural thickness. Gastroscopic examination revealed small focal lesions in the non-glandular part of the stomach around the margo plicatus of the greater curvature, with a score of II/IV Equine Gastric Ulcer Syndrome. Blood analysis revealed anemia, hypoproteinemia and neutrophilia. Treatment was palliative and was determined based on the initial findings. Despite intensive medical treatment, the horse was euthanized due to progressive deterioration of its condition. Post-mortem necropsy and histopathology revealed an adenocarcinoma with osseous metaplasia within the left dorsal colon.