Oi! Get off my land!

[Roadkill101]

I like to have some justification as to why an outsider or any other odd critters (for the region) show up for seemingly no reason. I find that a simple answer usually works best, such as The Insane/Evil Wizard/Cleric down the lane lost control over his summoned critter and said critter has found their way to the parties location more or less by chance. Assuming said critters aren't an integral plot piece, in which case there'll be a more detailed backstory.

For a similiar reason, I try to use only Random Encounter tables that have been tailored for the particular area in question.

I remember many of the old 1st ed. D&D modules were bad at providing any kind of explanation why some of the critters were where they were placed. In particular module B3 or B4 (it had a purple cover) had a nice story about a pyramid in the middle of a desert where the pyramids inhabitants were the descendants of the of a now vanished civilization. Below the pyramid was an underground city with more of said descendants living. However the lower levels of the pyramid were populated with all kinds of fantastic critters that didn't really fit into the story line by any rhyme or reason.

 

[TheAuldGrump]

Hmmm, IMC they tend to appear near gates/portals, not on random tables. But opening a gate/prtla also opens rips in the universe that allows outsiders in. (Generally creatures of up to 1/2 the CR of the critter that came through the gate, with none more than 1/4 the CR of said critter.) Outsiders and elementals popping up is a sign that something is wrong.

The players haven't figured this out yet...

The Auld Grump

 

[I'm A Banana]

In my Concordance campaign, the prime material plane includes heaven and hell and the elemental planes, so the demons and angels and such can quite literally walk from Hellmouth to your local forest if they want to. They don't have to cross planes, just use their legs.

In the campaign I'm developing right now, they need to be summoned or go through protals....and the very act of doing so is breaking apart the Material Plane.

 

[S'mon]

Elementals are native to the prime material in my game and carry the [spirit] sub-type, so it's not uncommon to find a pool that is inhabited by a water elemental or a geyser with a steam elemental in it.

I basically do this - an elemental or a mephit or a dryad is an "incorporated spirit".

 

[Selenim]

In my cosmology the prime has a few connections to the planes who compose it's "building blocks". I rarely use outsiders except when the location is special like the few static elemental vortices (an unusually active volcano for instance). Elementals tend to prefer their own plane's environment so that usually prevents invasions unless someone bothers to motivate them. For the most part they stay near the vortex since it's more "cozy" than the rest of the prime.

The plane of shadow also has a few connections that open up deep underground. These ones are usually temporary and the spot often moves around to reflect the morphing nature of the plane. This usually ends up trapping the shadow planar creatures in the underdark.

Of course there is always the classic major temple or other caster concentration that may import outsiders in the area as guardians.