For Teemo's unit ability that searches the top 5 cards and deals 1 damage for each card with Hidden, does a card like Ember Monk (which grants Hidden to other units but doesn't have Hidden itself) count?

Ruling: No, Ember Monk does not count. A card must have the Hidden keyword itself, not just reference Hidden in its text.

Nuances:

• "Having X" means having the ability X on the card itself
• Simply having the text "X" written on the card does not count as having that keyword