I agree, file a case now! This person is not just uninformed, I mean heck, the pony's country of origin is STAMPED on her. It says "Hong Kong" not "Greece". This is definitely someone trying to pull one over on you. She knows what a Greek pony is and even the Greek name for the pony, which means she knows enough about ponies to look at the feet. Even non-collectors know to look there.
And if for some reason your case is not found in your favor through eBay, file with Paypal.
And I agree with everyone else - stop communicating with the seller through messages. It's too easy for you to accidentally screw yourself over by saying the wrong thing. eBay has some weird rules about that kind of thing.
When you open the case, just state the facts as they are - this pony is clearly NOT Greek, it says so right on the pony. Also point out the differences between the pony in the picture the seller gave, and the pony she sent. Especially note the symbol differences, the obvious factory machine eye paint (instead of being hand-painted), difference in hair color, and body color.
After that, do not attempt to negotiate and do not respond to the seller. Just let eBay make the decision. And after that make sure to leave feedback before the deadline so others aren't scammed by this person.