How sad. R.I.P Horse.
Not quite sure what to make of it - On the one hand, the rider should've been keeping the horse under control on the road, on the other the police officer shouldn't have been putting himself into a position to hit it (if that makes sense?).
Moral of the story though - A horse on your bonnet will do a heck of a lot of damage. The British Horse Society is trying to educate riders on the importance of wearing high vis clothing - They have some horrific uncensored images of what happens when someone hits a horse.