So what does the monster eat?

There is a tv trope called "The Needless," describing characters which lack one or more of the same physical or mental needs as humans. In AD&D, every monster had an entry for "diet." 3rd edition ignored all that and made numerous monsters into perpetual motion machines. The vast majority of the time this detail is pointless and nonsensical.

2nd edition elementals had a diet. Earth elementals ate soil, rocks, gemstones and so forth depending on their specifics. Water elementals drank clean water. Air elementals respired clean air. Fire elementals burned combustibles. More exotic elementals consumed exotic food.

3rd edition and its derivatives introduced a preponderance of monsters that do not to eat, breathe and/or sleep. Pathfinder, or 3.75 in some circles, is a particularly annoying offender. It employs backwards world building where the fluff is shoehorned to fit the weird idiosyncrasies of the rules. For example, extraplanar creatures by the rules do not need to eat. Despite this, every other extraplanar creature is described as hunting and eating for pleasure or just because. Why even give them a digestive system? This is stupider then simply having them hunt and eat for survival.

For that matter, aging is generally only ever mentioned for player character races. It almost never comes up in typical campaigns due to the scales involved, but occasionally spells turn up which can age targets. There are no guidelines for what happens when these time spells are cast on monsters without aging tables.

So in my campaigns, every monster eats, breathes, sleeps and ages unless their lack of such is important to their concept. Exotic creatures may combine these needs, such as air elementals needing to eat air. Most of the time monsters will not stay around long enough to need resupply, so their vital statistics may be defined on the fly when necessary.

A few examples follow (subject to change since it does not come up often):

• Constructs have variable power requirements, but they do need external power. Those described as "powered by magic" are specifically powered by the magical wind of artifice, which may be manipulated or interfered with by brown magic. Other rely on less direct sources: biomechanoids require combustible fuel, clockworks must be wound up, robots must recharge their batteries, etc.
• Undead do not require food and drink in the conventional sense, but whatever animates them is continually deteriorating and must be replenished from an outside source. Liches and mummies are able to sustain themselves by tapping into the winds of magic, but other undead must feast or pursue their passions. Mindless undead slaves are unable to forage and must be sustained by periodic maintenance from their master.
• Angels are sustained by the ambrosia and nectar produced in Heaven. Angels on missions may have only a limited supply before they need to return, but may be able to rely on other sources such as faith or divine fire.
• Demons are sustained by desire, suffering and bad karma; the precise diet varies by the demon. Some are carnivorous, subsisting on the life force within flesh (preferably full of terrified adrenaline). Other consume wholly abstract diets: liquid suffering tortured out of damned souls, sinful desires tempted out of mortals, etc.