Mhmm I'm not sure about this. I can see why this would be useful, having the option to thrash or kick your enemies with your horse sounds useful.
I'm guessing that at least in one point in history people have trained, or permitted, horses to kick, thrash, and bite when in close combat. That's probably the natural reaction of a horse anyway, well apart from running away.
There is one thing though, staying atop a kicking or thrashing horse wouldn't be all that easy, so perhaps it could be a mid-high level perk in the riding skill.
Also I'm not sure how easy it is for a horse to kick whilst wearing barding (armour), wouldn't the weight impede it?