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)