"A Wizard Did It!" and leave it at that, but I, for one, prefer my settings with a bit more internal logic.
For Urbis, I ruled that all those creatures which look like they couldn't have possibly evolved in an Earth-like environment... in fact didn't, but usually came from other planets in the same solar system, implying that there was travel between the planets in the distant past and these creatures represent invasive species to the local ecosystem. This works well enough if you want to keep those creatures around as an occasional novelty. Still, there is another approach one could take - what if these creatures are not the invasive species, but humans (and elves, and dwarves, and...)?
Let's imagine the following scenario. We start out with a fairly standard D&Desque fantasy world with all the usual tropes and a fairly usual Earth-like environment.
Unfortunately, that world is doomed to die. In 10 years or so, the sun is going to explode, or a similar catastrophe of equal magnitude which is going to leave no survivors. And everyone knows this. Civilzation is still able to function, but the impending doom puts a damper on things.
Then one of the wealthier nations comes up with a plan - evacuate everyone to another world. Sages scry the multiverse, and while most worlds they find are too hostile, there is one fairly "nearby" (in magical terms) which is suitable - a terrestrial world with breathable atmosphere, with plants and animals digestible by humans.
And vice versa.
However, life there evolved along rather different lines. Instead of the birds and mammals and reptiles, the flowers and trees we are familiar with, the ecosystems on this world are extrapolated from the strange life froms of D&D history. With some limitations - if I want to publish this, I would have to limit myself to creatures published under the OGL. In particular, the primary resources would be:
- The Pathfinder Bestiaries
- The Tome of Horrors Complete, since that book has been able to collect all sorts of classic D&D monsters due to a special deal with Wizards of the Coast.
- No "Earth-like" creatures - no recognizable mammals, birds, reptiles and so forth.
- In particular, no recognizably humanoid creatures, as this would present seemingly relatable people and take away from the alien atmosphere of the setting.
So this is the world which is chosen by the inhabitants of our doomed fantasy world. Magical portals are being built - their cost is vast, equivalent to the wealth of a city-state, yet when the very survival of one's civilization is at stake then the cost must be paid. It is now Year Three of the colonization effort, and various nations have established beachheads and a ring of rapidly growing settlements on this alien world which struggle to make this land fertile for their crops so that they can feed the teeming, desperate masses pouring through the gates. Some of the gates have fallen to orcs and other marauding monsters who are now fleeing to this world as well. But the biggest challenge is the alien environment itself - its many inhabitants, many of which are deadly and many of which are not, and few of the refugees know enough to tell the difference. A number of seemingly intelligent creatures have been encountered, though nothing the refugees would recognize as native civilization - so far, although explorers have found long-abandoned ruins in remote regions...
The PCs can be peacekeepers solving the problems among the settlers, merchants seeking to exploit the riches of this new world, leaders trying to carve their own nations into the wild, or exploers uncovering the secrets of the land.