Many Dungeons & Dragons settings are old, post apocalyptic worlds. This was done to justify the existence of the abundant dungeons filled with loot that draws adventurers to plunder them. But what of the origins of the overwhelming number of species of monsters? Between the official monster books and d20 material, there must be tens of thousands of them. Here are three ideas on where they came from:

1. The Greeks where correct: monsters are the result of the gods punishing mortals for their sins. As some gods are on a monster creating kick and mortals not learning their lessons, new creatures continue to appear all over the world.
    
2. Monsters evolved on many different planes and those found on the PC’s home setting are the result of invasion over many centuries. People moving between the planes bring seeds, spores, eggs and adult monsters home for various purposes, much of which is magical research. The monsters adapted to local conditions, possibly interbred and have since run riot when the old civilization(s) fell to ruin.
    
3. A wizard did it. Old, cliché and very true in some worlds, this idea seems a little silly until one considered what the ancient world was like. Back then people were more powerful and had access to magics that could reshape reality. These people were not afraid of their creations for even the most powerful could be brought to heel with ease.

   The nastiest creatures were developed for the hunt. Initially it was things like trolls, hydras and wyverns but those were too weak for the greatest heroes. It turns out that power wasn’t the key, rather they found that intelligence provided the best sport. That is why dragons were created from giant lizards and ogre mages can shapeshift. Shapeshifting wasn’t terribly popular except to those hunters who wanted to be occasionally confounded. They wouldn’t use magic to discover their quarry, but rather pit mind versus mind and track them via their behavior. The result of course was the monsters for sport becoming the worst predators of mankind and their allies. They sharpened their abilities long, long ago.