I don't know if this is really considered a bug or if it is intentional - or if it was just forgotten.
As player, you can change your armour by clicking with an according armourpiece on the slot where you have already placed an armourpiece.
This is NOT possible with the horse armour.
I don't know if this is very important - it just takes a few clicks more - but I wanted to report it anyways, for the devteam to judge whether or not this is good as it is.
In case you need proof, I've made a video where you can see that it is not possible.