First thing I would try is warm water and soap with a washcloth/soft toothbrush.
Next, nail polish remover. I don't remember if you want acetone or non-acetone though.
If not that, a Mr. Clean Magic Eraser. Don't overdo it though because I've read that after a lot of rubbing it can be abrasive.
If all else fails, try this from the MLP Trading Post:
"Get a large pot of water and place in a scoop or two or Oxy-Clean or something along those lines. Let the water come to a boil and then toss the pony in. Let her or him relax. This is spa time for them and they're getting a lovely ex foliation. After a few minutes pull the pony out with a pair of tongs. Grab a nearby towel and give them a good scrubbing. Most of the dirt and dust that has been absorbed into the plastic should come off. Ink pen should come off too! The only thing this will not do is remove things like permanent marker or glue. Sometimes the pony might get flattened or come out mishapped. Relax put the pony back in the water and she or he should pop back into place. I have tried this on Princess Ponies and the medallion didn't change or nothing but that may just be dumb luck. I have not tried this on so softs at least not yet....so if that works tell the rest of us! As a fair warning I also recommend that after about and hour or so of cleaning your ponies like this it is a good idea to wipe off your ponies with some cool water to get the Oxy-clean residue off of them. Otherwise the pony will look like it has been playing in Comet or worse....... And make sure you clean your cleaning area too the powder will show up after about an hour or so and will look like powdery white spots."
Good luck!
-Em
