Make sure to take their heads off before you clean them! The lady who gave them to me was trying to fit little surprises in them for people to find. I think she gave up because most things wouldn't fit (and I pointed out that putting gumballs inside a mildew covered pony might be kind of gross), but there's probably still something in there.
She's a really nice lady and these ponies had a lot of sentimental value for her. She really wanted to do something nice for everyone. I just didn't think that maybe someone might clean them without taking their heads off.
Besides that, I cleaned a few myself and some of their insides are really gross!