While the majority of Equestria is a strangely static environment, requiring the attention of Ponies to alter the plant life, weather, and even the cycles of day and night, the Everfree forest is the exception.
An exceedingly dangerous exception.
Don't be fooled into thinking that the 'natural' growth of plants and changes in weather makes this place anything like Earth. The Everfree forest is, magically speaking, somewhere between a toxic waste dump and a nuclear disaster area.
Deep within the forest is the site of the final battle between the Princesses and Discord. While it ended in his defeat, the immense outpouring of his unique brand of chaotic magic during the course of the fight left an indelible stain on the magical energies of the area. In the years that followed, the Ponies were forced to abandon the castle that would have been the capital of the new kingdom, and resettle several towns far afield when the forest began to spread at an unnatural rate.
The name of the place itself is a clue to its origins. Reputably, Discord's last words were screamed in defiance, that this place, at least, would remain Ever Free of the Princesses' desire for peace and order.
Today, a magical barrier prevents further growth of the forest into Equestrian land. A series of small forts house the spells along with a garrison of Ponies who maintain a physical watch for any dangerous creatures that might slink out. Unicorns tune the barrier and maintain a constant monitor over the local magical fields.
Despite the dangers, the Everfree forest sees a great many visitors. There's always something new to discover and pick apart, and ponies have made their fortunes bringing back unique treasures created in the strangeness of this place. Armed guards frequently escort logging expeditions, foraging parties, and the occasional tourist with more courage then good sense.
When I went hunting for food here, I traveled alongside heavily-armed Pony patrols. No matter how tough you think you are or whatever prior experience you may have with hunting, doing anything less is practically suicide.
The forest consists of three rough sections. The relatively 'safe' outer layer is a stretch of forest, mostly ordinary beyond the extremely rapid growth of the greenery. Things become increasingly worse the farther in you travel, where the usual dangers of untamed wilderness are concealed, enhanced, and warped beyond recognition by magic.
The Deep Wood is an unstable, marshy wetland that holds a documented assortment of horrible ways to die, from magical parasites, giant carnivorous plants, and harmless-looking 'normal' animals that secrete lethal poisons. Beyond that is the wide array of actively predatory monsters. Every one of them is different, they all want to kill you, and sooner or later, there's an entirely new batch mutated by the Deep Woods and given a fresh set of lethal tricks to end you with. By the time you start running to safety, the landscape will have already changed beyond recognition.
And if you're stupid enough to go looking for the center of it all, the Heart of Chaos, well…no human has yet tested their resistance to magic against it. Even if you were to survive the experience, the border patrols have the orders and authority to kill-on-sight any mutated creature they perceive to be a threat. Whatever or whomever it may have originally been.