Creature Inventory
If you click on the Inventory tab in a creatures properties you will see at the bottom the following screen:

The list comprises of what the creature can be equipped or rather where. Click on the Edit button and the following screen will appear:

Now lets give our creature some armour. To do this click on the plus sign beside Armour (under the Items tab - bottom left hand pane) and then click on the plus sign next to the types of armour that will enable you to drill down to some actual armour. Left click on an armour of your choice and then drag in to the chest field of the Equipment tab.

If you want the item to only appear in their inventory then drag it to the Item tab in the upper left area.

Now rinse and repeat for each field in the Equipment tab, if necessary and your creatures equipment may look something like this:

You can click each item field to make it droppable or possible to pickpocket the item from the NPC (would try and go against having the chest, boots, etc from being pickpocketable). If you are unhappy with a piece of equipment then just left click on it and hit your delete key.

Beside the Equipment tab is the Armour Set and the Properties tab:

To find out more about the Armour Set and the Properties tab click Here.