When trying to use the replaceitem command to give mobs shields in the slot.weapon.mainhand slot, the command fails. Tried this on both skeletons and zombies, does not work. However, the command does work on players.
This is a bug exclusive to 16w21a onwards, probably due to the changes with dispensers now auto-equipping shields to a mob's offhand.
I tried this on multiple worlds, doesn't seem to be specific to the one I was using earlier. I summoned a Zombie, then ran the command /replaceitem entity @e[type=Zombie,r=5] slot.weapon.mainhand minecraft:shield whilst standing right next to the zombie. The game then displays "Could not replace slot slot.weapon.mainhand with 1 * [Shield]".