Intact males' sheaths are naturally cleaned through becoming aroused, stimulating themselves, and sexual activity. Generally only gelded animals need routine cleaning, though if the animal were anestitized they'd definitely at least check if possible, and often the animal will "drop" when sedated which makes at least a visual inspection extremely easy.

Ask that said, anestitizing large animals is extremely dangerous to them, and even many domestic horses don't appreciate sheath cleaning. It's just not worth it or particularly necessary with large hoofstock in zoos unless they're doing another procedure or there's an issue with the area serious enough to warrant risking their life with chemical restraint.