A saddle.
only some....
You can but it is not recommended. They can be difficult to get out of the stirrups if you have a fall and you could be dragged by the horse.
If the rider is riding the horse they wear a bridle and a saddle and if they are outside they wear a flymask and / or a fly sheet or nothing and if they're in the stall they will wear nothing or a horse blanket.
You can wear casual clothes with a horse riding helmet.
This is no more dangerous than if the horse is barefoot.
Horses wear horse shoes on the bottom of their hoofs.
a horse costume.
kicks
For it's back. it's spreads out the pressure, so when the horse has a back problem you don't put so much pressure on it. Oh: and it's a shock absorber. Think about it: it you wear a gel pad in your shoe the pressure will also be spread on the lenght of your foot, and so there will be much less pressure on the ground.
They don't wear anything but their fur.
A horse will wear a horseshoe that is small and sharp.
Yes, a horse has a back bone.