It was the Only logical Outcome, really
Celestia is a spider. This is the one immutable, immovable principle that the land of Equestria is built upon. That their immortal ruler, on whose eight shoulders rests the fate of the World and the path of Sun, is a giant, eight-legged, six-eyed spider, twice as tall as a normal pony.
This is the one truth Equestria stands by, and it's what shapes its culture and the lives of its citizens.
There in the city, by the street near the bakery, is a shop of fake prosthetic legs, plastic, wood and metal harnesses and coloured pieces ponies wear on their backs to simulate the image of their deity, anywhere from cheap hanging sticks to complex mechanisms of fully functional spider legs.
There, in front of the library, a boutique where one can get two extra sets of eyes painted on their face, and for a small extra price get a pair of fake fangs to display.
Eight the main streets of the city, as are the legs of the immortal goddess who moves the Sun, their structure centred around a centre and the connections between them in the shape of a web, their ruler's sacred gift to ponykind.
All through the town, cobwebs cover every corner, every hole, every passage, unbroken, and ponies walk around them not to break them. Spiders are the animals consecrated to their Princess, and it is a crime to harm them. Many bring them food and shelter, in hope of receiving good luck.
There in the school foals are thought to worship and respect Celestia the Spider, who raises the Sun and brings life to Equestria, and they learn of her great powers and of her marvellous adventures.
There, at the centre of town, is the temple, where ponies gather to pray to the spider who rules over the Heavens. Eight are the marble columns, and six the orbs at its centre, of gold or silver or opal or other precious material, depending on the town's wealth.
There, near the fountain, an old pony sits on a box and preaches that there was once a time when Celestia was not a spider, but in fact a pony like the rest of them. He is laughed at, insulted and made fun of, as every pony in Equestria knows this is falsehood, the rambling of an old and crazy fool, and that Celestia is a spider, and that is fact, immutable and irrefutable truth.
In her tower, sealed away from the rest of Equestria, Twilight muses over her books and memories, and feels her mind slowly slipping away. Celestia is a spider, they say. Celestia is a spider, they chant. And that is the truth, they claim and exclaim and proclaim. And she knows, somewhere deep inside, that was not the truth once. But can she be sure? Celestia has been a spider for so long, centuries, millennia even. Can she trust her memories, when all history books state that Celestia is a spider? Does it even matter?
And what if she wasn't? Is it not then the next logical step in a pony's ascension? She tried to understand it, to replicate it. She tried, but she failed. Why? It's driving her insane, her memories conflicting. She sees an image of Celestia, staring in a mirror and saying It would appear I am, in fact, a giant spider. Was it a dream? Was it all a dream?
There, on the mountain, is Spiderlot, the capital. Some claim it once had a different name, but no pony knows of anypony that lived to see its other name. And if Celestia the Spider doesn't say so, why should they believe it wasn't always Spiderlot?
Creature from all around the world come to visit the capital, some hoping to see Celestia the Spider, some even hoping she will choose them.
There in the capital is the castle, and in the castle are tall windows of stained glass, detailing the major events of the history of Equestria. There, a blue alicorn sits and stares at them, confused and feeling like she forgot something. But the windows clearly show how she turned into a wasp when trying to overthrow her sister and bring forth eternal night, and how could she ever doubt them?
There are many more windows. About how Celestia, with the help of her not-spider sister, defeated Discord by wielding the Elements of Harmony. How before that they faced against Sombra in the Crystal Empire, and though Celestia's incredible powers as a giant spider far outclassed the tyrant he still sealed the Empire away with himself once defeated.
The Empire then returned, eventually, and was freed thanks to Celestia's student. A very talented pony, Twilight Sparkle, though she never managed to reach her teacher and ascend into spiderhood, remaining a mere alicorn like the Divine Spider's sister. Some say envy drove her insane.
The Empire still exists, and its citizens too worship Celestia, the heavenly eight-legged creature who pulls the Sun through the skies and grants light to the land. They are not ruled by a spider, but they still have temples dedicated to Celestia. And Princess Flurry swears it, that for as long as she can remember Celestia was a spider.
Other windows adorn the walls of the castle. Of how Celestia banished her sister-turned-giant-wasp to the Moon. How she chose her student, and she and her friends purged Luna of her waspness. Of how Twilight later defeated Discord. More recently, of how Celestia permanently defeated Queen Chrysalis by trapping her in her web and imprisoning her in the castle.
There in the castle's halls are great threads of web weaved by Celestia. The guards and maids do not touch them, as they are sacred. And because they're also extremely sticky, leaving anypony who touches them permanently stuck. Not for long, though, as Celestia will feel their presence on her web and come take them away. Some throw themselves intentionally at the web, sacrificing themselves to please their goddess.
Others are taken in the night by Celestia herself, moving silently on the walls, and disappear forever. No one sees them again or knows where they went, but their disappearance is celebrated with happiness, as they were chosen, and it is an honour to be chosen by the Spider Deity.
There in the library, the access forbidden and impossible for any normal pony thanks to a series of strong webs holding the doors shut, are many cocoons hanging from the ceiling, and inside them the ponies and creatures Celestia has chosen and captured. They are kept alive by food Celestia brings them, and at their request she might even bring books or other things. But she never talks, and they never see anything other than her shadow as she crawls over their cocoons.
They spend their time chatting, telling stories, singing and occasionally playing chess. One of them in particular excels at that, a strange creature who calls herself Chrysalis and claims to be the old Changeling Queen of the legends. No one knows how long she's been there, they all say she was already there when they arrived. But she says many strange and weird things, like that Celestia once wasn't a spider, so no one believes her anyway.
There, in her room, comfortably resting on a web, is Celestia herself, in all her spiderful glory. She reads a book, holding it with two of her legs, while a third holds up a cup of tea she occasionally brings to her fangs to sip on. And she is, without the shadow of a doubt, a very large spider.