Funny

I'm by no means saying this is a definitive win. Just that setting a spear (I perhaps should have mentioned this in the previous comment) is a historically proven method of countering a charge. You might still die but I think with a proper spear odds might actually be in your favor if only slightly

In this very skilly hyper specific scenario where you are tasked with fighting a horse to the death, but with the stipulation that you are given prep time, I believe you could find and properly whittle a passable enough stick to function as a rudamdntary spear for the purpose of bracing against a charge, and that's assuming you don't have some particularly sharp metal/rocks on hand with some basic lashing materials. If the horse pulls up and tries to canter around the set spear you could hoist it up and now you have range. If it pulls back for another charge? Set it again. You don't have to thrust with it, just hold it point out and wait for the horse to either impale itself or get tired. That's how boar hunts work after all.

Like yeah I don't like your odds lmao but its not nearly as doomed as you might think imo