Lethargic dog with anorexia and pain

History of Present Illness

An 11-year-old castrated male Havanese dog is brought to the clinic by his owner for evaluation. He has been lethargic, anorexic, and yelps or bites when picked up. The referring veterinarian noticed a mass near the anus on physical examination. The owner tells you the patient has been lethargic for about a week and is not eating voluntarily. The owner has been giving food and water to the dog by syringe. The dog now has difficulty walking, and the owner seeks a diagnosis.