Hidden
Definition
Hidden mobs require you to be affected by detect hidden to be able to see them (some equipment can affect you with this permanently, like the shades), in which case they look as normal, just with a little (Hide) tag in front of them. If you can't detect them then they won't be listed when you look in the room, but you may see a message like 'Someone arrives from the east' (although equally this may be someone who is invisible or wizinvis. Hide doesn't conceal players from other players, but it will still fool any mob without detect hidden. In either case, fighting will remove the affect, or a player can cancel theirs by using the vis command.
Hidden items are something quite different, they're items that are undetectable unless you successfully search for them.