- D&D 5e Monsters Manual
Well, there is another story.
As the behir's color, shape and lighting breath can surely prove, the first behirs were not "created" by storm giants, but shaped by them from defeated blue dragons.
Most dragons were simply slain for daring to resist the (superior) giant race, but blue dragons wielded lighting, and the storm giants could not forgive such a sin against their elements.
First, their wings, arms and legs were ripped off, and then they were thrown, like agonizing worms, into claustrophobic tunnel-prisons, never to see the sun again, never to taste the freedom of the endless skies.
Eventually, the dragons used their magic to grow small legs that would allow them to move and escape, but the predicament drove them mad.
Turned into multi-legged abominations, behirs hide their shame deep into the earth, hating everyone and everything, especially well-formed dragons and the light of the sun.
If you see a behir, it is unlikely to let you live.
On the other hand...
Some people do not believe that behirs are created by accident. These scholars say that, as fire giants shape metal, storm giants shape flesh.
They believe themselves to have in their blood a genetic power that makes them perfect. Every other creature is subject to experimentation... and improvement.
They hate dragons, of course. And they hate magic. And fey. But they love science, their "blood-shaping" art more than anything.
Some say all giants are the result of their experimentation - maybe even themselves. Other think the lower giant to be primitive, or decadent forms of storm giants. The stupid hill giants certainly look like a debased version of their nobler cousins.
But it could be worse.
Fey Curse. The elves remember when the fomorians were among the most handsome of races, possessed of brilliant minds and unrivaled magical ability. That physical perfection did not extend to their hearts, however, as a lust for magic and power consumed them. The fomorians sought to conquer the Feywild and enslave its inhabitants, claiming those creatures' magic for themselves. When the fey united to defend their realm, the fomorians fought them and were subjected to a terrible curse. One by one, the giants fell as their bodies were warped to reflect the evil in their hearts. Stripped of their grace and magical power, the wretched horrors fled from the light, delving deep beneath the world to nurse their hatred. Cursing their fate, they have ever after plotted vengeance against the fey that wronged them.
Or so the elves say.
But the "curse" was something different entirely.
See, "the most handsome of races, possessed of brilliant minds" were certainly storm giants.
But they messed with something storm giants were not supposed to - maybe magic, yes, but more likely defeat. Storm giants despise the weak. After being beaten by the fey, these vile cowards were reshaped into something hideous by their giant kin, and forced to fight the fey until the end of time.
Like behirs, they hide in deep caves and avoid being seem by others - specially giants.
Magic can do nothing for them. Only the storm giants can reshape them back - if they ever fulfill their duty of slaying all the fey.
Golems are obviously the giants' creation too - artificial beings made of earth, metal or flesh, resistant to magic, immune to shape-changing, and made to the (twisted) image of their progenitors.
The dragons and the fey are beings of magic, air, beauty, and freedom. They do not always get along, of course - but they rely on each other to survive and thrive. Sometimes, they are like like lions and gazelles.
Giants - who cannot understand magic or reach to the skies, no matter how tall the mountains their climb - hate the fey and want to destroy them.
Humans? Eh. They will often fight for one side or the other, but are not essential in the grand scheme of things. Both giants and dragons seem to think they are clearly inferior and are not ashamed of using them as plaything, slaves or food.
The Abyss is a grey, rocky, and dead place, full of wide spaces, where almost nothing seems to move.
It might be older than time.
Some say that is why the dead - and undead - turn to grey, and why the graves are marked with stones.
And probably why giants give so much importance to their boulders, and why the dragons and the fey - in defiance of death and decay - are so colorful.
In the middle of the Abyss, lies the Worm.
The Worm's colors are grey, yellow and purple.
You will hardly find dragons of these colors. Instead, dragons are the colors of the sky, the clouds, the trees, or living blood - things the Worm usually misses or despises.
As fire giants shape metal and storm giants shape flesh, the Worm shapes time.
Deep in the Abyss, it sends forth their sons and minions to conquer the earth, or manipulates people to worship it.
The Petrifying Serpents such as the medusa, cocaktrices and basilisks, that want to turn the world into a stony wasteland to please their master.
And the wingless grey angels that travel through time and space and seem to come from nowhere.
The nagas and yuan-ti, obviously - and who knows what else?
Maybe the undead that pull the living to the underworld, or the aberrations that mock nature.
If it looks like a snake, or reptile, or a dragon - without wings - it was probably created by the Worm.
Likewise, if it looks like something that was never meant to exist, is is probably Worm-spawn. Not hybrid animals, such as the beautiful Pegasus, a colorful pixie or couatl, or the giant dragon turtle that fights the worms of the sea - but something with tentacles, uncountable legs, twisted limbs, numerous eyes... or no no eyes at all.
Some say the Worm avoids fire, and its minions fear it, preferring to use poison and decay as their weapons. But some servants of the Worm can relish in fire too - is is just less common.
In spite of all that, the main power of the Worm is not creating monsters, but seducing the innocent.
Storm giants, serpent cultists, drow, mind-flayers and sahuagin - they might all be worshiping the Worm, wittingly or not, since so few people understand what the Worm is.
As they stare into the Abyss... the Worm will happily stare back at them, and show them they are special, perfect, superior to all others, no matter what they say.
The Worm will promise them a perfect world.
But, in reality, the Worm hates the world. And nature. And most colors. And the sky, above all else.
Because, as powerful as the Worm is, it can never leave the Abyss, like the dragons did in ancient times.
But - given enough time - it might make the Abyss swallow everything.
Further readings (some of my inspirations for this post):
* Dark Souls lore.
* Obscene Serpent Religion.
Moral of the story (if there is one): sometimes, a single origin for many monsters might be more interesting than monsters coming randomly from everywhere. Also, Evil versus Chaos might be more interesting than Good versus Evil.
All images copyright of Wizards of the coast.