When dealing with a world in which you've established a creation myth--no matter how loose said myth is--you have to justify everything.
I've given rationale for humans, elves, dwarves, and hobbits. Hobbits are the eldest race, a failed attempt at creating an all-purpose slave and pleasure race, but their penchant for cultivation made them useful. Humans were an improvement on hobbits, but while they were jacks of all trades, they were masters of none, and the Ancient Ones needed (or wanted) grand structures and crafts dedicated to them. So they fashioned the dwarves out of stone, to mine and create, and like the underworld in which they live, dwarves are as cold and hard as stone on the outside, but burn with the angry fires of the earth within. Eventually, the humans staged an uprising against the Ancient Ones, and their malcontent threatened the largely defenseless hobbits. To ferret out the leaders of the human rebels, the Ancient Ones created the elves, dark, mysterious, and seductive, to infiltrate and root out the troublemakers. This accomplished, the humans firmly back in their place, the elves were set to guard the hobbits so that agriculture could continue. And though exposure to the peace-loving hobbits tempered the elves somewhat, many still head up cults dedicated to the return of the Ancient Ones, and few amongst the other races will readily trust an elf.
The rationale for 99% of the other monsters in the Wasted Lands is simple. The Ancient Ones are evil creatures of black, alien mind and driven by chaos. They experimented and created hundreds--even thousands--of races to use as playthings and just for their own amusement. Some monsters were failed attempts at creating other races--orcs and goblins, for example, were early attempts at elves, who ended up cast underground to become bitter and angry at being seen as failures. Others, such as kobolds, gnomes, gnolls, trolls, etc. were just random tinkering. Still others, such as giants, were created just to see how humans might react to the threat. Finally, there are those creatures such as dragons and demons, which are not native to the world at all, but were dimensional travelers who came, found intelligent life and natural resources, and stayed for reasons of their own.
Does this somehow make humans, elves, dwarves and hobbits races of destiny, with a greater fate than the other species? Not at all. The future is a great black hole in the Wasted Lands. There are no prophecies--at least, not on any grand scale--of what may come. There aren't even solid records of the past, as the Scourge of the Godswar wiped many of these clean as the burning of the Library of Alexandria and the Church of Rome's widespread destruction of so-called heretical writings cost us many of our own ancient records, albeit on a far greater scale. Make no mistake, when the gods themselves war in the physical plane, it creates unimaginable turmoil, devastation, and cataclysmic alterations in every level of existence, from great kingdoms to the individual lives of insects.
Indeed, certain great quests in the Wasted Lands could be to find caches of ancient religious and historical writings, much as those we today have found in the Nag Hammadi Library or the Dead Sea Scrolls.
It's feasible and even probable that some of the monster races have their own creation myths, but these will be dealt with if and when it becomes necessary. For now, they exist simply because they do--no one can truly fathom the thought processes of the Ancient Ones.