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If you get prep time you could set up some traps.
Assuming both sides see it as a fight to the death, the horse will also engage so you could just run away into a bunch of traps. All you need is for the horse to injure a leg in one trap and it’s done for. I think even just some holes with a couple spikes would be enough to injure and maybe even sprain an ankle.
Without prep time you’re pretty doomed, I think your best bet is either climbing up a tree to buy you some prep time to make a spear out of the branches or worst case diving in, aiming to do damage to its legs (unlikely) and hope you are able to get out without being trampled (unlikely)
Good luck reaching an unwilling horse’s eye balls