The story goes like this: sometimes, when an animal (or even a humanoid) dies, something goes wrong.
Perhaps the complex web of conduits and vortices that make up the spiritual highway of the cosmos confuses the disembodied spirit. Or maybe an Ethereal cyclone or astral+storm drives the soul off course.
However it happens, sometimes spirits end up in the Inner Planes instead of the Outer Planes. Instead of becoming Petitioners, they clothe themselves in the raw stuff of the Inner Planes, becoming something very much like an Elemental.
That's how you end up with horses and dogs made of fire, or medusas made of lightning. It's rare, but it happens.
That's an animental.
Planescape Monstrous Compendium Appendix III - "Animental"
Manual of the Planes (D&D 1.0) - Appendix II: Creatures of the Inner Planes
Manual of the Planes (D&D 3.0) - Element creature