Having thought long and hard about it, I think that while monsters GENERALLY don't poop or pee or fart or such, that doesn't mean they aren't ABLE to do it.

What makes the difference between having to "hit the can" and not is the food.

As I probably explained in another comment (coz I'm very invested in these little details XD) monster food is converted automatically into energy upon consumption, which goes for both monsters and humans.

This means of course that the monster food doesn't pass through the digestive system, hence why Frisk never needs to go to the bathroom the entire time they're in the Underground (which may have lasted overall about a day or two, I think, maybe even less than a day).

The plant monster in Grillby's comments on human food passing through the entire digestive system and then wanting to try it sometime.

We don't know how human food affects monsters, but given that they did originally live on the surface where there was human food - and given the strangeness of the Underground and the Barrier - we can assume that monsters do have digestive systems like humans, but they simply live on monster/magic food that is easily consumed.

And because of the magic food, they never need to go to the bathroom hence why there aren't any bathrooms.

I think most monsters (with a mouth) would be able to eat and digest food normally, though it would need some getting used to.

Some monsters though, would not be able to eat human food, either because they have no mouth or their physiology is too complicated/flawed to do this. By the latter, I'm referring to ghosts and skeletons. Ghosts are incoporeal and would not even be able to touch human food, let alone eat it.

As for skeletons, they can touch human food, but trying to eat it would simply have it fall through their ribcages onto the ground. They have no organs other than the SOUL (if that can even be called an organ), so monster food is the only food they can eat.

It also seems that some foods retrieved in the dumps (human food that came from the Surface) also have become magical and converts to energy upon consumption. My guess here is that the Barrier maybe has an effect on everything in the Underground, including food. Normal human food is turned into magical monster food, strange plants grow without sunlight, there's snow, and the gravity and atmosphere can be manipulated through magic.



(Perhaps the reason why Frisk wants so badly to leave is to go to the bathroom LOL JK but why not)