Perhaps you've run it through water (puddles, streams, etc) regularly and your oil pan gasket has hardened with age, allowing water to seep into the pan?
I'm not sure if the Mule is water- or air-cooled, but if it's water-cooled as you hint, yes, it could also be a leaking head gasket.
Clean the rust out of the oil pan, and replace the oil pan gasket. Assuming you have no trouble out of the Mule, wait until your next service interval and drop the oil pan again to inspect for rust. If it has re-appeared, have the head gasket replaced.
One more thing: ALWAYS make sure you re-torque the oil pan bolts to the factory-specified torque setting, as too little torque might not give a good enough seal, and too much torque might cause the gasket to pinch and fail. Either way, you end up with water in your oil pan, and probably an oil leak as well. If you haven't invested in a torque wrench for this purpose, do so now. Sorta pricey, but worth it if you're going to do your own maintenance.
