It is my understanding that it is the sellers responsibility to make sure that the item is in the appropriate packaging and arrives to the buyer in the condition that it was in the pictures. Now, I am worried that if I file a "Item Not Described" case or leave negative/neutral feedback that the seller will reciprocate and leave negative feedback on my account. I am tempted to just un package the pony and eat the extra cost and not have to deal with this seller right now....
You are correct. It is the seller's responsibility to make sure the item arrives to the buyer safely and in the stated condition. They can't just pass the blame along to the post office. Even if the damage was caused during shipping it would be their responsibility to initiate contact with USPS, not you (though if they didn't buy insurance the USPS isn't going to do anything).
You don't really need to worry about seller retaliation. Sellers cannot leave buyers negative or neutral feedback on eBay. If they leave positive feedback with a negative comment it can be removed (I believe this is against eBay policy or used to be, though I can't find where it says this anymore). The worst thing they can do is block you from purchasing from them in the future (though you may not want to anyways based on this experience).
How you proceed depends on how badly you want to keep the pony and how much of a headache you want to deal with. If you do not want the pony because of the damage to the box, open an "Item Not as Described" case. I can pretty much guarantee eBay would find in your favor. A seller's return policy doesn't really matter to eBay if what you got doesn't match the description. I'm not sure whether the seller or buyer would be responsible for return shipping, I've read mixed things.
If you want to keep the pony you have a couple of options. One is to leave neutral/negative feedback to warn future buyers, block the seller and move on. Or if you prefer you can just not leave feedback at all. I would personally probably at least leave a neutral based on the seller's rudeness and lack of accountability. Unfortunately, sellers are not required to provide partial refunds. When a seller refuses this solution the options are basically force a return through the INAD case or keep the item/leave appropriate feedback.
I'm really sorry this happened to you
I had a similar experience with a MOC figure set that was thrown into a box at an awkward angle with no protection (no bubble wrap, air pockets, paper, nothing) and the plastic part arrived completely separated from the back card. The seller apologized and offered to let me return it but because it was a rare set I had been searching for for so long I just decided to keep it and left neutral feedback for the poor packaging.