This bug is VERY odd. Skeletons (or at least the ones that come from skeleton horse traps) will sometimes have their armor completely change when you exit and reload the world. One moment they'll be wearing a gold chestplate and helmet, and then if you quit and rejoin they may be clad in a full set of enchanted chain armor.
See video for repeated demonstration:
I have never seen this bug occur with anything other than skeleton-horse riders, though I can confirm it happens even if the skeleton is no longer riding the horse. No idea if it happens with other mobs or even with other generic skeletons. This might have something to do with how the skeletons