PHB page 146 says that donning and doffing a shield takes 1 action for PCs. However I did not find anything about monsters.
For example, a goblin (MM page 166) wears both a leather armor (AC 11 + Dex 2) and a shield, making its total AC 15. In similar fashion, Helmed Horror (MM page 183) wears a plate armor (AC 18) plus a shield, making its total AC 20. In fact, every monster with a shield seems to have the shield bonus of 2 as a part of its AC.
Is it safe to assume that the monsters with shields always wear them, or should the AC initially be -2 until the DM decides to make them spend an action to don a shield?