Contrary to common belief, the beasts did not create the world, but merely designed the many creatures who came to live in it. At first they started with the small things who writhe and crawl. Those were humble and pious, and sylvan was delighted by their many-colored chitin and elytra, but they soon became bored, and so moved on to making creatures who ran and swam.
those had amazing potential, but spent most of their lives unaware of themselves or the world around them, so the beasts taught them to dance and sing, to craft both stories and objects of beauty
and with that, the first people were born
They do not actually call themselves great beasts, or even think of themselves as important despite the fact that they are the closest thing their world has to gods. They're just the world's oldest friend group and were pretty much just playing around. This took a very long time, as they would be doing other things. For example, after the invention of the more complex creatures, sylvan fell asleep for a very long time. When they woke up and saw how much fun tabial and gidaa were having teaching the animals things they simply had to get in on that