At various places in the planes, rifts of special tears appear and allow the mundane creatures of one plane to intermix with the creatures of other planes. On these occasions, it is most often not the terrible beasts such as demons or angels that pour through, but much less powerful (but not harmless) creatures mirroring the beasts of the natural realm.
These are just a few of the possible creatures available, but they should be a good starting point to create other elemental animals.
Medium Fire Crab
Rappan Athuk Copyright 2012 Bill Webb, Frog God Games