This is how Twilight explains the Origin of the Symbiote goes in My Little Pony: Equestria Girls: The Symbiote Siren.
[Twilight looks at various photos of the Symbiote in its human form]
Twilight Sparkle: The way you described it, I think the Spider Siren sounds an awful lot like... the Symbiote.
Pinkie Pie (EG): Not the Symbiote! [whispering to Fluttershy] I don't actually know what that is!
[Twilight picks up a book and it shows the Symbiote in his spider form standing on a rock]
Twilight Sparkle: The Symbiote was an amazing but dangerous creature who had the power to turn ponies to the Dark Side. But to maintain this power, it had to attach itself to the pony in order to feed off its anger. The more of this negative energy it consumed, the stronger it became and the farther it could spread its army of dark ponies.
Fluttershy (EG): I don't think I like this story very much.
Twilight Sparkle: If the Symbiote had its way, it would have multiplied and conquered all of Equestria. But a certain Star Swirl the Bearded wasn't having it. Rumor has it he found a way to get rid of the Symbiote. He banished it to a world where he thought its power would be gone for good. And I think that world is this world where you guys live.
Sunset Shimmer: But that was long ago. Why is it just surfacing now?