I've been doing some work on monster design, and stumbled on a weird problem with monsters needing to breathe. In my current monster write up, I don't intend for breathing to be something that is required. It's a magical creation, and it could survive with no oxygen if needed.
However, I've noticed that specific monsters have "Amphibious" if they can breathe both air and water (See Aboleth MM pg. 13). Also "Hold Breath" when they can hold their breath for long periods of time (See Plesiosaurus MM pg. 80).
Humans, which I'm positive have to breathe, have no statistics to indicate they do. Normally I'd say that breathing is inclusive, and a stat block will alter if that is the case (IE, they can breath water too). However, a good portion of undead/constructs shouldn't have to breathe (example, ghost) but there is no indication that they don't have to breathe.
When do I need to put something in to the stat block regarding when a monster can/can't breathe? (Assuming that the article would be published for distribution, so other people who did not design the creature might need to run it).