Kitty litter, baking soda or fresh air. All of the above are pretty much the best way to remove a smell/scent which is lingering on a pony.
Place the pony in a sealed container along with a dish of litter or soda to absorb or neutralise the smell. After a week or so, give the pony time to breath in a location where there is fresh air and then repeat the process until the smell is gone.
You dont need something as dangerous as bleach for this to be honest and you then run the risk of the pony smelling of bleach.