Going to be tough- very tough especially if the hair wasn't groomed over several days during the actual drying process. Are you sure the hair was standing BEFORE packing?
Best chance is to use a slightly damp towel/rag right along the hair roots of the neck mane for 24 hrs. Make sure you aren't getting the rest of the hide too wet. Use a hair comb to back brush the mane hairs until they want to mostly stand. You then need to "card" as we say in the taxi world, the hair to dry standing up. You can use masking tape/blue tape to try and catch all the hairs in the standing position, use strips of cardboard placed on each side with T pins etc, I even use old hair combs to leave stuck between the hairs. At any rate, you need the hair standing for several days to dry. Each day, comb the hair back up and re-card.
After all that, you MIGHT get them slightly standing. If they weren't originally trained and dried at mounting, there's really not much chance.
A little hairspray and hairdryer might also help at the final stage. I've even tried using a woman's flay iron to iron the hairs up.
Unfortunately, all with less than desirable effect UNLESS I was able to keep them standing at the initial mounting phase. If the hair roots have been dried and locked into place in the laying position, then you'll never move them. All you are really doing is bending the hair at the root to the up position thus they will eventually fall
