Random hitting with riding on horse

Dear Mojang.

I search next strange bugs.

If i ride on horse  i have random hits and my horse too.

I don´t know where he came from but with a horse it is unplayable.
 
 It´s not all.
If i ride on horse and go uphill with him, i have same strange hits.
It usually hits me about five hearts without armor.
 
Thanks for reading.