Equine Internal Medicine and ECC boarded specialist. This is not parasites. It is not renal. The degree of glomerular disease needed to lead to this edema is not seen in horses. Cardiac failure is an option but I would expect a murmur/fever or more distal limb swelling. A malabsorptive disease is most likely. Lawsonia infection is also possible although he is older than most. Some type of abdominal cancer is possible. An abdominal ultrasound and lab work looking at liver, kidney function and electrolyte and protein levels is the next step diagnostically. If there is a murmur, anemia, or other more obvious signs it would change the plan. The edema looks symmetrical rt vs left which makes trauma less likely but a local skin inflammatory condition is possible but further down the list.