They say if you're going to boil a pony head, body and all, you need to remove the head first, otherwise it creates a vacuum in the body and it kind of implodes.
To boil the head to soften it up for removal, I'd suggest just putting a bowl of water in the microwave for a minute or two, and dipping just the head and neck in the bowl for a while, then try taking off the head.
These are just ideas, however. I've never successfully removed the head of a G3, but I only tried once and gave up after a tug.
The ponies out now shouldn't have glue in their neck seams since their heads turn, but all the old G3s have tons apparently.
Good luck!