I don't think a horse would choose melee if it can run, and if you prepare for range (multiple javelins), you have an advantage. A gokd hit on the more sensitive parts of the legs can knock it down, and while it keeps running you could collect your projectiles and keep trying. Persistence hunting can wear it out, even if it's faster. Once it's on the ground, finishing it wouldn't be as dangerous any more if you stick it from outside kicking range.

If it does fight up close, yeah, that'd be terrifying, but the range advantage of the spear would give you an edge still. Physically, I think I could pull it off.

Emotionally, I'm pretty sure I couldn't handle it. I'd go down, paralysed at the thought of hurting it.