I've always found the Everfree Forest to be the most interesting part of the show's mythology. I love that they give just the barest of hints as to the backstory of it... Setting a castle that likely belonged to the two Princesses deep within it, making it the most frightening environment to the characters within the show, and then giving it a name as positive and benign as "Everfree". It all just whispers something happened here.
They've tamed their entire world down to a micromanagement of nature itself (the stress of maintaining such a grip on every aspect of it is a likely culprit in the high rates of mental imbalance among the pony set. They're like an entire species made up of frazzled postal workers.) But not that forest. That forest they have just written off... warning ponies to simply never go inside there.
No actual story behind this one... Just an attempt to draw Zecora that spiraled out of control a bit. Seems fitting, considering the subject matter.