but why do i stop because he hit my horse head from side? It doesn't have sence at all.
That's the mechanic. If a polearm over a certain length hits the horse's head it will reared. There's a lot of things that doesn't make sense in this game, and this one is not the worst.
I am telling you the truth. If he stop me,he hits me one time ant take 30% of health,and stun my horse,then he hit my horse again and stun him,and again and kill my horse,then i fall and while i am on ground without block he stabs me and take me half of health. Everything happends in about 15 seconds. Long spear is so fast,not to mention that war spear can outreach my heavy lance and stab even faster.
Long spear is fast? What the... Also, you can't be reared/stunned twice in a row, so you're obviously making things up. If he rears you and only takes 30 % you're doing something wrong by dying there because the next hits won't have any speed bonus at all and you should be able to escape easily. I've done it enough on my cav alts myself.
If you get outreached by a war spear I guess you're charging head on? If you tilt your horse to the left side while stabbing/couching to the right you will outrange bamboo spears and long awlpikes, not to mention war spears. Only weapons you can't outreach is long spear and pike.