The Everfree Forest is a heavily wooded area on the outskirts of Ponyville. Once a lush forest, it's trees have been corrupted by Hoarfrost, rendering it a dangerous frozen waste land. Instead of beautiful, it is blizzardy. Instead of spectacular, it is SNOW-tacular. While it was already dangerous during the Second Era, as of the events of Source, even more dangerous plants and creatures now call it home.
As of the Fourth Era, it is colloquially referred to as "The Everfreeze".