Unfortunately, I don't know anyone who can repair this. The only thing I may suggest (hopefully someone can verify) is to buy some epoxy sculpt and mix a color to match the pony. You might be able to fill in the hoof to make it less noticeable.
Well, I think 'repair' is a tricky verb to use here; will she be exactly like she was before? No, because ponies are made of blown plastic, and you can't really re-create that without creating a whole new pony in a factory, I think! But I think if you mixed some apoxie and sculpted her a new hoof, you could sand down/trim the jagged edges to make it flush, and paint the new limb to a very close match. To the trained eye, there will likely always be a seam where the plastic ends and the epoxie/clay begins; but on display, from a few feet away, it could be made to look very close to original.
I'm not practiced in this kind of repair, but I know some people are, and hopefully one of them who is open for commissions will see this! Best of luck, BethyVic! I'm so sorry about your pony!