Depiction in the series
The Everfree Forest is first introduced in Friendship is Magic, part 2, when Twilight Sparkle reads the book The Elements of Harmony: A Reference Guide. It specifies that the Elements of Harmony's last known location is in the ancient Castle of the Two Sisters, and that the castle itself is located in what is now known as the Everfree Forest. Twilight Sparkle and her friends enter the forest in order to retrieve the Elements.
When the ponies venture for the first time into the forest, Applejack surmises that "It ain't natural; folks say it don't work the same as Equestria". In Bridle Gossip, Applejack, Fluttershy, and Rainbow Dash elaborate that the plants grow on their own, animals take care of themselves, and clouds move without pony intervention.Fluttershy, who lives in a cottage near the edge of the forest, tells the Cutie Mark Crusaders in Stare Master that the forest is "much too dangerous" and is "filled with far too many strange creatures". These include timberwolves, the manticore, a notable sea serpent, Ursas, parasprites, dragons, and the cockatrice. Timberwolves are shown in both Family Appreciation Day and Spike at Your Service, while the rest are respectively featured in Friendship is Magic, part 2, Boast Busters, Swarm of the Century, Owl's Well That Ends Well and Stare Master. The abundance of monsters explains why the ponies are wary of the fact that Zecora makes her home in the Everfree Forest, citing it as one of the reasons they fear her in Bridle Gossip.
The forest is mentioned more than once as a place of banishment. When Rainbow Dash panics over her impending performance in Sonic Rainboom, she fears that she will be banished into the Everfree Forest for her failure. Similarly, in A Bird in the Hoof, Twilight suggests that she and Fluttershy will be banished and possibly imprisoned in the Everfree Forest for kidnapping Princess Celestia's pet phoenix, Philomena.
The Everfree Forest also contains many types of flowers, poison joke being among them. In Bridle Gossip, Zecora cures Twilight Sparkle and her friends by using medicinal plants.
Granny Smith mentions in a flashback in Family Appreciation Day that she ventured into the Everfree Forest one night when she was young and discovered zap apples. Shortly after, she is chased back to her farm by timberwolves.
Season threeIn the episode Too Many Pinkie Pies, Pinkie Pie ventures into the Everfree Forest to look for a magical pond that she recalls from a nursery rhyme. She pushes through dense brambles and unexpectedly falls down a hole but ends up in an underground cavern that houses the Mirror Pool. She recites, "And into her own reflection she stared, yearning for one whose reflection she shared, and solemnly sweared not to be scared at the prospect of being doubly mared!" and steps through the surface of the pond, successfully cloning herself. This process continues until there are a sizable number of clones, who start overwhelming Pinkie's friends. When all the clones are sent back, Twilight seals the entrance to the Mirror Pool with a boulder.
In Spike at Your Service, Spike accidentally enters the Everfree Forest and is attacked by Timberwolves, and saved by Applejack.
In season four's two-part premiere, the Everfree Forest begins to grow out of control, causing bramble vines to overrun Ponyville. To prevent the vines from overrunning all of Equestria, Twilight and her friends return the Elements of Harmony to the Tree of Harmony, which lies in the heart of the Everfree Forest. It is later revealed that the vines are the work of Discord, who planted plunderseeds long ago when he first fought Celestia and Luna. The magic of the Tree of Harmony kept the vines from growing properly over the centuries, and it was only recently that the spell was weakened enough for the vines to grow outside the forest. With the Elements returned to the Tree, the protective spell is reinforced and the vines destroyed.
In Castle Mane-ia, the main cast delves deep into the Everfree Forest and head to the ancient Castle of the Royal Pony Sisters. The Castle and the Everfree Forest are seen again in Inspiration Manifestation, Twilight's Kingdom - Part 1, and Twilight's Kingdom - Part 2.
In an alternate timeline in The Cutie Re-Mark - Part 2, Zecora uses the Everfree Forest as a base for her resistance against the changeling invasion. Queen Chrysalis finds the hideout and engages Zecora in combat, forcing Twilight Sparkle and Spike to flee.
Depiction in the comics
In the My Little Pony: Friendship is Magic Issue #27 and #28 story arc The Root of the Problem, creatures and living vines from the Everfree Forest invade Ponyville. It is caused by a tribe of deer living in the secluded forest village of Thicket. When a construction company destroys forest land to make room for an amusement park, the deer use their unique magic to make the Everfree Forest expand outward, covering its bordering lands and villages in plants.