A non armoured horse shall suffer if they bump over a tincan. Just imagine it.
And I have used horses in real life and they are not tanks neither cars neither unstopable force of nature.
I think that if they lose like 1/20 of his life if the bumped wear full plate armour is fine.
Man, think it. 
If you implement very small damage on horses that can represent how they can get tired or exaust of hitting and bumping too much.
And there are very different kind of horses and weights and strongness but that is another story.
Remember that war horses were taught to also to kick frontwards while they run. So they wouldn't always bump with their "chest". It could also be funny if it were possible to knock rider off the horse. Without horse or rider dies.
Though if someone gets hit on a horse while riding it's usually instant death.
