So now that we're considering a critter's place in the Monster Ecology, what's next?
What do the monsters eat? How do they subsist?
Does it eat?
If so, what does it eat? Here are six options. Roll a d6, or pick one.
1) Normal Food. This monster eats normal food. Animal, vegetable, or fungal sources of food are required. They may eat other monsters, local vermin, or incoming adventurers. They may farm their food or be dependent on feeding from other monsters in a symbiotic or master/slave relationship.
If they reside in an area with limited food supply, they may be extremely efficient (and thus not need a lot of food), sleep a lot (and thus eat a lot, infrequently), or some combination of the two. They may both eat and reproduce off of their victims, like slime monsters frequently do.
2) Exotic Food. They drink blood, or eat brains, or nectar and ambrosia (classic for godlings), or survive off the life energy of their victims, leaving them shriveled and aged or weak-constitutioned after a few strikes. It might still be organic, or comes from organic creatures. Feeding off the creatures may or may not destroy them. If it doesn't, they may keep farm stock, willing donators, or simply leave near weaker fodder creatures. If it does destroy them, maybe they need to trade for sources or live a peripatetic life.
3) Minerals. Your classic gem-eaters, rock-eaters, or metal eaters. They may eat the rock fish out of the lava rivers of the underearth, or lurk in shadows waiting for adventurers in plate armor to walk by. Which rock they eat can matter - a gem-eater might pass rock as waste, or need to mine the gems out. Rock-eaters may be desired pets for miners or hated enemies.
4) Mental Energy. Your psychic devourers. They may eat all mental energy, or just survive on certain emotions. Devouring thought is common for psionic creatures, while devouring emotions is classic for undead types. They may encourage thoughtful environments by hanging out near odd displays of art or puzzle rooms, or destroy all organized thought with their rampant feeding.
5) Souls. They eat souls, straight up. They may kill you and then devour your soul as it flees the body, or suck it out directly as they attack. They may be vulnerable to eating a poison soul (whatever that may be!) or need innocent souls to sustain life. They may be unable to eat innocent souls, requiring them to corrupt their food before they can eat.
6) Can't Eat. The monster wants to eat, but can't. Re-roll, or pick another entry - that's what it tries to eat. But it can't sustain itself by it. Either it is unable to gain nourishment from it, or it cannot eat enough, or its appetite only increases as it eats more and more.
This covers monsters like the mermecolion of myth, which was born with a lion's head and an ant's body. It would starve because its lion head craved flesh but its ant body rejected the sustenance. It would starve quickly, but not before spending a short-but-angry existence.
What if it doesn't eat?
If not, why not? Is it a purely magical or supernatural being, with no need for sustenance? Does it have an external power source (broadcast energy, from some central location) or an external dependency? Can this be interrupted, or is it wholly self-contained?
Robots on broadcast power have an external source. Golems with a scroll of instructions inside have an internal source, and do not eat. Do they depend on mana? Undead created by magic might require magic to live, and be unable to survive in a no-mana area (like a GURPS No Mana Zone or inside an AD&D anti-magic shell). Those created by hate and unholy energies might survive anywhere, or they may only be able to reside places of residual death or on defiled ground.
Another option switch is that the monster doesn't have to eat, but may in fact eat for pleasure or power. Determine what they eat normally, and then decide if it's a requirement or not. If not, does eating give them some kind of benefit? Do they die without food, or just weaken?