Definitely contact cross-contamination, could be from just about anything with printing on it but I'd put money down on the MIB packaging. The shape, intensity and placement is wrong for hair as the source. Smooth poly-bag doesn't take paint very well, not nearly as easily as vinyl. Moral of the story is don't store your ponies touching anything that has ink, paint or dye on it. You can wrap them in acid-free tissue if you want to be completely sure there's no touching. Plastic bags are not recommended because while they do stop ink transfer, they create microclimates that can cause humidity problems.