Just to clarify, a cocoon is NOT a pupa. A cocoon is the silken envelope some caterpillars spin around themselves BEFORE they enter the pupal stage. The pupa would be the actual insect inside the cocoon. Few caterpillar species that pupate below ground spin a cocoon.

I think that is a pupae of a saturiidae moth. Probably Eacles imperialis or one of the two Citheronia species in your area. I doubt that it is a Sphingid. What trees are nearby where you found the pupae? Also, you could keep it in a small storage container in the fridge untill May and see what hatches out of it.

I am so appreciated for your comments. you have helped more than you know!! as for nearby trees, the two above the site this was found are ash and pine. ill definatly use a container but i must ask, should i use a small amount of soil and store out side in its natural environment? this sounds like a perfect age appropriate project for my 2yr old whos b-day is dec.20.

If it's ash and pine then your pupae probably belongs to Eacles imperials, or possibly Citheronia sepulcralis.

Anyway, you could either cover the pupae in the same dirt or leaves or whatever you found it and keep it inside a container outside. Except outside, you risk animals getting into your container to eat your pupae, which wouldn't be good. Or you could roll the pupae up in a piece of toilet paper (not scented) and place it in a plastic tub without airholes and keep it in your fridge.

Here is some info that might help.
http://www.butterflyschool.org/teacher/raising.html