It's on his right side high up on the rib cage just behind the withers and about 2 inches down from his spine. He doesn't seem to be in peril, and appetite is good. It's hard and about the size between a large marble and a golf ball. I've got an animal hospital appt. for him 5:30 Wednesday. I lost Yogi and Eli within a year of each other not long ago. I can't take it if it's not a fatty tumor or something treatable!!!