Poisons cause specific chemical reactions that result in specific symptoms. What is poisonous to a Human physiology may be inert to a Lizard physiology, for example. I would expect a supernatural or an otherworldly entity to have alien or magical physiology. So, poisons not working on monsters seems perfectly feasible.
I came here to basically say this, but with an addendum.
I say that poisons should work on monsters...
however, not normal poisons like you'd use against humans. You'd have to find specific (and mostly very rare and in most cases widely unknown) poisons for that specific monster. What would be poisonous to a manticore might not be poisonous to an insectoid, for example.
And, of course, some monsters should not be effected by poisons at all, like undead or ghosts or demons.
But to expect a standard generic "poison" to work on monsters seems a bit silly, IMHO.
I'd personally roleplay it. Your players want a poison that paralyzes a manticore? Sure, but they'll have to have a mini-adventure just to find someone who knows how to make that poison, and then another adventure to find the ingredients which are all very rare and strange, like silver, toenail cuttings from a giant and an orcs snot.
After they've done that once, though, they'll know how to make the poison themselves, and might use it again in the future as long as they can find the super-rare ingredients, of course.
EDIT: And, of course, some monsters are so close to human/dwarf/elf physiology that they'd be effected, but they'd need greater doses... like an Ogre, for example. They're half human half dwarf, so logically most poisons would work on them, but they're massive and tougher, so maybe you'd have to use ten times the dose for it to work.
