If you come upon any first batch from the 80s generation 1 My Little ponies, or knock offs, that need a good cleaning and look like they might have rust or other gunk inside them you will need the following-
A thin but sturdy small knife, I used a small fillet old one
Optional cutting glove so you don't poke yourself
Optional dish gloves to keep your hands soft
An old toothbrush
Basic dish soap or Zote laundry soap
for really nasty insides Oxyclean laundry additive
The first step will be getting the pony's head off. Trust me, this makes it much easier to dry. Start by using elbow grease to twist and turn, you may need to soften the pony a bit with some heat. You should be able to see the seal around the neck coming apart. In stubborn areas that's where the knife comes in to use but don't poke too deep. The neck has a male knob that fits into the body.
Give the pony a good soap and water bath and toothbrush scrub. If the insides are okay set aside to dry, fix the hair, whatever.
If the insides are rusty or moist fill a sink or container with hot water and a couple tablespoons of Oxyclean, it does a good job of neutralizing the ick. Make sure to get the Oxy water inside the pony and swish it around and use the toothbrush to loosen the gunk. The water will get nasty. Hopefully one round will do it. Afterwards rinse and give another soap and water scrub.