When I first started playing this wonderful game known as D&D I was the 'first on my block' as it were to have the materials so naturally I became a DM out of necessity. I drew in friends to play and thus began my life-long hobby.
Like most from my era I had only one Module to begin with (the famous B2 Keep on the borderlands) and my players and I played the heck out of that thing. We went everywhere on that map and I tried my hand at building dungeons (The Caves of Chaos still makes me wince). Truth be told I sucked but my first dungeon did have a forgotten temple with goblin cultists. They managed to kidnap the Castellan's daughter and the players had to carve their way through to save her. When they reached the cliche dark temple with 'tapestries with images to their dark god' one player piped up 'what does the images look like?' and this line of questions lead to the naming of my first 'dark god' right there on the spot. His name was 'Vulgaar'. (Yeah like I said I sucked back then) It satisfied my players and we moved on.
Throughout that year I picked up some other modules. I got, the A Series the G series, the D series, Q1 and T1. These introduced me to the concept of a world and I used them to shape my own world (As the Greyhawk gazetteer was no where to be found). These modules also introduced other dark deities (Elemental Evil and Lloth just to name a few). I got Eldritch Wizardry which introduced me to move evil to throw at my players. Thus was born my world.
Fast forward a Decade or three and I joined the Greyhawk chatroom (Greytalk HERE is the link). I always loved Greyhawk because through its early adventures I built my world. I was welcomed and I got to rub shoulders with many of the folks that wrote for things like the Oerth Journal, Living Greyhawk and the like. (It was also nice to not be the oldest Geek in the room too :) ) It was during a reading of some Living Greyhawk material that I saw something that made me go 'hmmm'. It was the origins of Saint Kargoth the original Death Knight. In all of the Material on the Demogorgon his 'other name' Amon-Ibor is accredited to the Oerdians yet in this origins piece it says clearly Kargoth after not getting to be the leader of the Knights of the Great Kingdom took refuges in a ruined FLAN temple to Amon-Ibor. So I reasoned the Flan who predated the Oerdians were the ones who used that name for the Demogorgon. Now usually most things like this end there because getting in contact with the author was nigh impossible. Not so on this because the author of this and other such pieces, Gary Holian, was a regular visitor to the Greyhawk chatroom! So posing this little conundrum to him personally and stating my theory he agreed that the Flan must have named him (It was this conversation that lead me to postulate of an undiscovered 'Golden Age of the Flan and the concept of Naming but that is for another time.)
What does this have to do with the title of this post? Well, this rambling is me showing you faithful readers how I stumbled along before getting to stage of being able to polish things.
'From Primal Fears and Ageless Dark crawled the Sibilance and swam the Shark. From Dire Crags and Forests Dread came the Wolf and the Hungry Dead. Four Great Beasts, Nightmares All will ever hunt us and seek our fall.' Excerpt from the Arrenite Sacra Derkas (The Lightless Psalms)
Before the rise of mortals the Abyss was a different place, home to things no mortal mind could conceive these creatures of Dread Chaos waged war against the Immortals of Law. As an expression of Chaos itself the Abyss is ever changing so when the first mortals came into being so to came creatures that would be called demons. The first demons though were not born of mortal sin as few such things reached the Abyss in those days instead they were born of the primal terrors that filled the collective mind of early mortality. Amon-Ibor the Sibilant Beast, Dagon father of the hungry sea, Kerzit the Winter Maw, and Yeenoghu foul lord of Ghouls these four earliest of demons make up the Great Beasts found in Arrenite lore. The greatest of them became the King of Demons later known as the Demogorgon. The later Lords and Princes of Demonry born of mortal sins could never match these four beasts in power.
One would think the Great Beasts would end mortal life before it could spread but with Primal Fears comes Primal Hopes and from these hopes were born the protectors of mortality, the four-fold Mother and the four-fold Father, the aspects of the seasons each embodied the hopes of early mortals as manifested in the world around them. These two beings were the genesis of Druidism and would evolve into the patrons of the first priests. (Again this is a subject for another time)
Gleaning from existing gaming material is how the Great Beasts came into being. While the published origins stand in opposition to what I wrote above I am reminded of the words in the forward of the first DMG 'This is your game if you don't like a rule change it, if you don't know a rule make it up.' So here they stand, the Great Beasts.
When I started to make the Great Beasts I looked through the reams of demon lords and princes searching for things that were recognizable but still alien enough to be something early man might imagine. Obox-ob and Jubliex were just too alien while Graz'zt and Orcus were too human. I knew the Demogorgon would be a member because I liked the idea of him being the first demon so I used him as an exemplar. Dagon, a creature mixed of Squid and Shark fit perfectly. Kerzit and Yeenoghu also fit the bill being basically humanoid with an animal feature. I stopped at four because I used the seasons as a measure. I modified their origins and polished them up and I had the evils between the Alien and the Human.
Being a world-builder rather than using pre-made worlds forces me to adapt things to fit my world but there's no reason someone who uses pre-made worlds can't adapt things to their liking. Were I to use Greyhawk for example the Great Beasts would still exist but be the foes of the Flan or in the Forgotten realms they'd be the enemy of the Elves.
Though these are worlds built for our use we don't have to use them as recommended because there is no warranty to void. They are our worlds to use as we choose. Codify your evil and see where it takes you.