This question was prompted by recent discussions on dragon breath weapons and cover. It occurred to me that a dragon could potentially use the Ready action to use its breath weapon once a certain number of foes were within its area of effect (waiting for a hiding enemy to come out of full cover). But what if the enemies stayed in cover for the entire round (if the trigger never happened)? Could the dragon then use its breath weapon on its next turn, or would it have to roll a d6 to see if it recharged (rules for recharging found in MM, p. 11)?
My inclination is to say that simply readying a breath weapon doesn't use it up, since nothing in the rules specifically says it does. But on the other hand, the closest equivalent in the rules would probably be readying a spell (since, although a dragon's breath weapon isn't magical, it does have an area of effect and saving throw). And when you ready a spell, you use any resources that the spell consumes, whether you end up releasing it or not.
Does a "Recharge X-Y" ability need to recharge after it is Readied (but not used)? Or does it only need to recharge after it is triggered?