Are creatures automatically immune to their own abilities?

Question

It seems creatures using elemental damage are not automatically immune to that damage type since they have an Immunity entry (see Fire Elemental or Hell Hound).

What about poison and disease? The Poison Frog has a poisonous bite but no indication that it's immune to its own or that from other Poison Frogs. The Poison rules do not make any such mention either.

Should I assume two Poison Frogs fighting it out would poison each other with their bites? Also, a Poison Frog biting itself (it's, um, a little crazy) would poison itself?

Abilities of interest: Poison, Disease, Curse, Elemental damage, Petrification

Creatures of interest: Oozes (with acid attacks), Mummy (mummy rot curse), Basilisk/Medusa

Answer 1

I never met a general rule saying a creature is immune to poison just because it's venomous.

Besides, real life venomous animals aren't either.

There may be a difference between venomous animals (who poison their preys, like snakes) and poisonous animals (with a poisoned flesh to avoid predators, like some frogs or the famous fugu), but I don't think there's many poisonous animals in Pathfinder bestiary, since there's very few adventurers feeding on monsters corpses.

EDIT : Concerning your edit :

• Oozes are immune to poison, sleep effects, paralysis, polymorph, and stunning
• Mummy, as undead, are immune to disease, so they're also immune to Mummy rot, which is both a disease and a curse (and I don't think there's something like immunity to curse) (source : paizo FAQ)
• Medusas aren't immune to anything, like a wizard isn't immune to Fireball

It is IMHO laudable to worry of the apparent silliness of an acid-covered ooze not immuned to acid (other than its own, apparently), but you're making a mistake trying to fit Pathfinder monster rules into real-life zoology and biology, or even physics (or whatever science, really). I recommand you to either reskin/refluff things you as a DM think unrealstic (but this is a really treacherous ground) or to go with it and don't bother with any real-life stuff. I'm pretty sure the latter will make you happier, both as DM or PC. Unless you're really into zoology and realism, then I would advice to go for an other game than Pathfinder.

Answer 2

Short and long answer, it depends. If you would find, say, a poison ooze, it would be immune to itself, but a snake rolling 2 times 1 would attack and poison itself. Aberrations should also be immune to their own stuff, unless its a template on something else, in my opinion.

Elementals being immune to their own element is just like the ooze example. HOWEVER magic trumps most of the time. I would argue for sacred flames harming an elemental (considering that the elemental is most likely evil).

Also, a tree would be harmed by a wooden weapon, metal golem being harmed by metal, etc.

Two poison frogs should poison each other.

If your DM wants something more rule-yyy, then particularities override generalities and as Trajan mentioned, the generality is that creatures are not immune to their own poison or other such effects. For things like aberrations and perhaps magical beasts, I would say its either mentioned or you all agree they should be immune or not. Else, DM calls it, like always.