>>2314557No, especially not a concernable amount. It's pretty hard to get water inside the head, you need to worry the most about the tail, which can rust, and you're more likely to mold the pony.
Even dunking or submerging quickly won't particularly necessarily hurt. It's keeping it in water or not allowing it to dry that will let it mold.
With that, don't use hold hair products on ponies. No hairspray or gel. Wax might be ok, but hairspray and gel can make the hair brittle and dry and you'll never get it fixed, and it can cause plastic deterioration.