Celestia is a Spider and That's pretty much It

One morning, Princess Celestia woke up as a giant spider. She did not appear to be particularly concerned by the news.

On one morning, without explanation, Princess Celestia woke up as a giant spider. Her guards, her sister, her once student Twilight and the rest of her ponies and other creatures of Equestria were rather shocked at the fact that such a transformation had taken place in her.

She didn't seem to mind it.