Mobile, quasi-sentient-and-upward trees aren't necessarily evil, but they're often hostile, because that's scary.
This is the third type of tree listed in Diana Wynne Jones's The Tough Guide To Fantasyland. Variations include, as quoted from the book:
Prehensile. These will grab you with their branches and try to eat you.
Mobile. These will quietly move about and get in your way. They may also try to eat you. In extreme cases they will attack buildings.
Poisonous. Avoid any drippings from these.
All these things at once. Run away.
These tend to contribute to Gaia's Vengeance and may populate The Lost Woods. A Plant Person or someone with a Green Thumb may summon these in battle. When facing these, killing it with fire is recommended. A good source of Paranoia Fuel: as something innocuous you see all the time, it really shouldn't be fighting.
Moreover, if you think about it, the concept of a mobile, fighting tree is actually a lot more worrisome than it might sound. For one thing, humanity has built a lot of stuff out of wood- like houses and boats. Notice a common theme? Those things are built to last. Imagine a being, made entirely out of that substance, who dislikes you and has strength proportional to that toughness. A living tree would pack one hell of a punch — and that's not even taking into account how much the thing would weigh, or the sheer number of limbs they have...
Hostile trees are also a source of humor, partly because of the absurdity of something seemingly so innocuous and ignorable striking fear into the hearts of men, and partly because "tree" itself is a mildly Inherently Funny Word. If you're lucky, they'll turn out to have more bark than bite.
Compare Plant Aliens and Man-Eating Plant. If you see lots of these, it might indicate that you are in Mordor or a Garden of Evil or on a Death World. If your trees are attacking you, it may be a sign that they're epileptic. Or not...
Also by Rumiko Takahashi, the anime (at least) of InuYasha had Ninmenka, a Youkai in the form of a peach tree who devoured humans, particularly favoring the flesh of sages. Possibly based on a creature from real Japanese mythology, this charming demon is also referred to as the Tree of Human Faced Fruit, which may make it a distant relative of the Death's Head Tree of Dungeons & Dragons. The name comes from its fruit, which bare ugly human faces in response to all the people it has eaten. Humans can eat these disgusting things to gain demonic power and longevity, and a Ninmenka that has eaten enough sages can produce a fruit of true immortality.
A less hostile tree youkai named Bokusen'on also appears once in the series to give some exposition on Inuyasha's full-demon side. Though peaceful, he's presumably still pretty badass since his branches were used to craft the unbreakable sheaths of Tessaiga and Tenseiga.
At the beginning of the Lost Children chapter of the Retribution arc from Berserk, Guts encounters a tree that had been used as sacrificial grounds for so many years the faces of the dead appeared on its trunk. At night, it awakes and wreaks havoc upon the bandits who'd been camping underneath it.
The Makaiju (The Doom Tree/Tree of Life for the Dub Watchers) from the Sailor Moon anime. A tree from Rei's shrine also becomes one when infused with a Demonic Seed thanks to Kaolinite and transformed into a daimon.
The X-Men villain Black Tom Cassidy used to be a guy with a walking stick that he could shoot death rays from. Then he got turned into an Ent. He's since gone back to being an Irish guy with a goatee and cane.
From Hell It Came - an entire movie about an evil and (barely) ambulatory tree.
The short horror/comedy film Treevenge is about a group of vengeance-happy Christmas trees.
Cthulhu. The protagonist nearly runs over something on a backwoods road at night, but we only see a glimpse of a spooky leafless tree. A similar creature later tries to reach him in his prison cell — these scenes were however cut to a minimum due to Special Effect Failure. Presumably the creature was meant to be Shub-Niggurath (The Black Goat of the Woods with a Thousand Young) from the Cthulhu Mythos.
Most likely it was meant to be one of Shub-Niggurath's Dark Young, as they are usually described as looking like walking leafless trees, only the "branches" are actually tentacles.
The Annette Funicello version of Babes in Toyland featured Gumps, animated trees with faces who captured travellers in the night and escorted them to the Toymaker.
The Guardian featured a dryad-like witch who sacrificed babies to a living tree. One scene featured chainsaw wielding guy going mano-a-treeo and winning.
The Harry Potter films contain the Whomping Willow, for obvious reasons. See below in Literature.
In Jumanji, one of the game-cards causes giant plants with grappling tentacles to spring up and spread out across the neighborhood. We don't actually see them catching people, but one did drag off a police car. Later a poisonous barb shot from it fatally wounds Judy.
Little Shop Of Horrors has Audrey II as the main foe. Though not quite a tree, the potted plant is easily the size of one by the time all is said and done.
In the parody film Trees, a 'Great White Pine' serves the same role as the shark from Jaws in killing and attacking people. And apparently, this isn't an uncommon creature...The sequel, Trees 2: The Root of All Evil also has killer Christmas Trees attacking the town.
Womaneater had a man growing a human-eating tree in his basement and true to the title, fed women to it.
The Ruins had killer vines that grew more ridiculous over time. Before long they were able to mimic sex noises to inject paranoia into the couples.
In Big Fish, Edward is attacked by trees while walking on the path in the forest alone on a road which had a warning sign, but he knows he doesn't die that way, so they let him go. He ends up in the town of Spectre right after that.
In The Cabin in the Woods, one of the options for monsters that could have attacked the heroes was "Angry Molesting Tree." Later, when all the monsters break loose, we see the branches of one grab a guard and pull it offscreen, presumably to stick its sticks where they don't belong.
The Ents in The Lord of the Rings, though they aren't so much overtly hostile as simply reclusive. They did get pretty riled up over Saruman cutting down their ancient forest to fuel his war machines. They also tend to distrust dwarves for carrying axes.
Then there's the Huorns; they're ordinary trees that have partially awakened and become "entish" - they drag dark mist around them, so you can't really see them move, but from the descriptions it seems that they have the speed of a bullet train when they get riled, and despite the "not see them move" bit, they are quite capable of eating(?) an entire army of orcs without trouble. Old Man Willow, mentioned in Book 1, is presumably a particularly nasty, but lonely example of these entities.
Old Man Willow may be a different being of a similar type, since he seems to have supernatural powers wider in range and scope than the Ents... or else just a greater inclination toward evil.
To give you an idea of how badass the Ents are — Trolls, one of Sauron's stock Giant Mook troop types, are corrupted imitations, weaker versions of Ents.
Pippin: "It was staggering. They roared and boomed and trumpeted, until stones began to crack and fall at the mere noise of them [...] Round and round the rock of Orthanc the Ents went striding and storming like a howling gale, breaking pillars, hurling avalanches of boulders down the shafts, tossing up huge slabs of stone into the air like leaves. The tower was in the middle of a spinning whirlwind. I saw iron posts and blocks of masonry go rocketing up hundreds of feet, and smash against the windows of Orthanc."
The Illearth War features Caerroil Wildwood, a sort of male Dryad. Fleshharrower's army is eventually swallowed up by the forest under his command. Since The Land is in many ways a darker, edgier version of Middle-Earth, unlike Fangorn he demands a price for his assistance.
The Whomping Willow from Harry Potter. Harry and Ron learn this the hard way in Chamber of Secrets when they crash Arthur Weasley's Flying Car into it. It comes up again in Prisoner of Azkaban when the Dementors make Harry fall of his broomstick during a storm - it ends up flying into the Willow, which tears it apart. Lupin mentions that it was planted during his own schooldays because he is a werewolf, and the tree was to conceal the tunnel he used to get to the Shrieking Shack every full moon where he could safely transform.. Students used to play a game where they saw how close they could get to it - this stopped after one of them lost an eye.
The original hostile willows from Algernon Blackwood's The Willows, H. P. Lovecraft's favorite weird tale.
A certain species of tree in Nyissa in the Belgariad. Not only is the fruit lethal and the bark poisonous to the touch, but the tree itself takes a sadistic pleasure in luring unsuspecting victims and feeding on them. Ce'Nedra, a dryad, collapses screaming in horror when she reads its thoughts. Polgara makes a point that systematically burning trees like that one is one of the very few things she and Salmissra agree on.
Edmund Cooper's The Expendables #4, The Venom of Argus. A tree on the alien planet Argus acts similar to the Xanth tangle tree. Instead of grabbing, its tentacles act more like harpoons, stabbing prey and bringing them to the tree's mouth to be swallowed and eaten.
The plot of a Men In Black novel involved aliens that vacation on earth. The aliens appear as trees that would eat just about anything by slicing up the creature (or person) with their roots and absorbing everything. Amusingly enough, the light from the Earth's sun was dangerous to them, as it eventually turned them into real trees (plum trees!).
The shambler trees (actually a mutually-supporting ecology of plant-like creatures) in the sci-fi series The War Against the Chtorr. The shambler tree and its carnivorous tenants harvest an area, then move on at a rate of a kilometre a day. "A Season for Slaughter" goes into unsettling detail explaining how such a tree could walk (it has to virtually become an animal) and how it feeds.
In Dan Abnett's Gaunt's Ghosts novels, the trees of Tanith were mobile, although very slow; the danger was that no path would remain in place. This is suggested to be Chaos taint. Although Tanith was lost the day the regiment Gaunt's Ghosts, Tanith First and Only, was founded, its member make marvelous scouts because of their ability to manage in these woods.
One Sesame Street book featured Cookie Monster discovering a cookie tree and trying to eat its cookies. The tree then grabbed him and menaced him until he apologized for being greedy.
In Dan Abnett's Warhammer 40000Horus Heresy novel Horus Rising, the planet Murder had trees on which the aliens threw dead bodies before they ate them. The trees themselves caused horrific storms that trapped Space Marines on the planet. (Fortunately, one Marine was so horrified by the desecration of the corpses that he blew up some trees, causing the storms to die down and so revealing they were behind the storms.)
G. K. Chesterton has a short fairy tale (The Tree of Pride) that tells of a time when all trees were this way. Then one of them started to eat the birds that nested in it...
Deathworld had the entire ecosystem of Pyruss being hostile. The brain bits seemed to be some overly psychic plants. Blowing them up just made the ecosystem angrier.
Trees in the Forest of Skund in Discworld can talk. They're not dangerous, just really, really annoying.
The path to Mordaga's lair in The Elric Saga is protected by a grove of 49 Elder Trees, each of which holds an imprisoned soul. When anyone approaches, the trees release countless leaves which drift toward the intruders, latch onto their flesh, and start sucking their blood. The trees can also lash out with sharp, sturdy branches.
In the second book of The Cleric Quintet, Cadderly is able to convince the elves to attempt the ancient and nearly forgotten ceremonies to awaken the trees so as to battle the orc invasion. Needless to say, the ceremony works. Unfortunately, the enemy quickly realizes that the trees are like siege towers, powerful and huge but also slow and flammable.
Though not really trees, and not mobile, amphistaff polyps in New Jedi Order are living creatures purpose-bred by Yuuzhan Vong shapers. They're (multi-)armed with flexible, blade-edged limbs, can spit venom, and eat just about anything. Their seeds/limbs are harvested by the Vong as personal weapons.
In the same author's Spellsinger series, the giant mushrooms of the Muddletup Moors prey on travelers with an empathic attack, then absorb their nutrients as they decompose.
The first section of William Hope Hodgson's The Boats of the "Glen Carrig" involved a land full of prehensile-branched flesh-eating trees. H. P. Lovecraft himself called Hodgson a master of scary writing.
Due to the war with the faeries, the world of Bones Of Faerie is filled with enchanted trees and plants that try to attack and kill humans.
David Drake has a thing about killer plants. The Jungle and Redliners both feature trees that will try to kill you in one way or another; in fact, there are several different ways a tree can kill in Redliners, including exploding to fling out armor-piercing spikes, spraying those who come too close with a fast-hardening (and acidic) sap, the bark turning out to be tentacles....
One of the stories about Jakub Wedrowycz features sentient walnut trees. They can't move, but due to radiation from the Chernobyl incident their walnuts have mutated into brains, so they can communicate via Telepathy and Mind Control people, and intend to Take Over the World. They try to enslave the protagonist, but he slyly slips them some weedkiller, then comments that their plan was doomed to fail within a few months anyway — walnuts fall off in winter.
In Sergey Sukhinov's Castle on Venus (part of the 21st Century Chronicles series), there is a forest of alien trees on Venus, theorized to have being brought there by a comet. Some of these trees are able to uproot themselves and act as animals. They are not sentient, though. While they are non-carbon-based, they don't shy away from trying to consume humans. Sure, they spit them out after determining that we don't taste right, but it's usually too late for the poor victim. The trees are very aggressive and attempt to destroy any manmade structure, including the road that goes through the forest. One of the popular vacations on Venus is the safari, which involves seeing and, possibly, hunting some of the animal-trees.
The willow trees of Chronicles of the Kencyrath can move autonomously much like the Old English legend. They generally don't attack humans, but do cause a fair amount of collateral damage including one poor soul who has a willow wand try to root itself in his flesh.
Killer trees show up in several of the stories of Manly Wade Wellman. One Silver John novel, The Old Gods Waken, had the Man In The Oak, a kind of undead tree-spirit, as its main villain, along with a grove of literally bloodthirsty thorn vines. He also did several short stories that featured evil trees.
Galaxy of Fear: Spore has an encounter with the vesuvague, a tree with mobile strangling vines.
In the fourth book of The Dresden Files, one of the villains sends a plant monster after Harry. He calls it a chlorofiend, because saying 'plant monster' sounds silly, but no one understands what he's talking about so he has to revert to calling it a plant monster. Murphy kills it with a chainsaw.
An episode of The X-Files appeared to involve trees killing people. However, this being The X-Files, a fair amount of what was going on and why is left up to the viewer.
An early Saturday Night Live skit, "The Killer Christmas Trees", played like a horror movie - opens with Garrett Morris singing an a capella "O Tannembaum" while a fir tree skulks up behind him, and impales him through the chest.
Needless to say, Doctor Who has had these, in The Keys of Marinus, and various other bits of plant-related mayhem over the years (e.g. The Seeds of Doom and Terror of the Vervoids).
And now the 2011 Christmas special has sentient Christmas trees. They turn out to be good, but (though YMMV) are rather nightmare inducing before you find this out.
A mild version appears in an episode of Star Trek: Voyager where it's played for laughs. Captain Janeway buys an alien plant without realising it's prehensile. "I went to water it and it GRABBED me!" — causing fans to quip that the plant was getting more action than Chakotay ever did.
The MC Bat Commander and Eaglebones Falconhawk find themselves trying to tie a ribbon to the top branch of a tree in The Aquabats Super Show episode "Uberchaun!". The tree comes to life and attacks them; eventually Eaglebones summons The Dude to tie the ribbon, and the tree becomes dormant.
The Haven episode "Roots" had trees that extend their roots like tentacles to tear people apart. They were animated by The Power of Hate between the feuding Keegan and Novelli families, and revert to normal when they make peace.
An episode of Angel had a demon tree that would capture people with its vines and drain their body fluids.
The Maples screaming "OPPRESSION!" at the uncaring Oaks, in the Rush song The Trees.
In an early 1985 Peanuts Sunday strip, Snoopy encountered a walking tree while playing golf:
Snoopy: Sometimes I think that as soon as I hit a shot, a tree runs out and stands in front of my ball.
The Kite-Eating Tree is usually just a tree that Charlie Brown has the bad luck of continuously getting his kite caught in, but sometimes it is shown literally eating the kites, thinking to itself, and grinning.
While the Kite-Eating tree does seem to sadistically enjoy eating Charlie Brown's kites, eventually Charlie Brown seems to enjoy it once he realizes that he's the only kid in the neighborhood who flies them and without him the tree would starve.
The adventure "The Wandering Trees" in Dragon magazine #57.
Monsters: treants, quickwoods, hangman trees, etc. These can get really creepy, like the Death's Head Tree, which resembles a willow that sprouts screaming, biting human heads as fruit (each of which is the head of one of its victims) and the Seedroach, whose "seeds" are animated cockroach-like constructs of woods that sting living beings and turn them into new Seedroach trees.
And of course there's the Wolf-In-Sheep's-Clothing, which only looks like a tree stump...
Something in the soil of Ravenloft turns all plant-creatures that grow there into homicidal killers, even if they'd otherwise be good guys. There are vampiric undead treants there.
Eberron is a setting where all the NPCs are of relatively low-level, so that the player characters are the ones who kick ass and take names. The most powerful NPC in the setting? A level 20 druid who's an awakened tree.
The recent release "Heroes of the Feywild" feature the Hamadryad as a player-character race. They're basically standard Dryads that are (for one reason or another) no longer bound to a specific tree, but still have the "plant person" look. An earlier book also released the "Wildkin", although that's more along the lines of "when shrub-elves attack".
Specifically, the are creatures and spells that will turn a Forest into a creature that's still a land, such as Awakener Druid.
Stirring Wildwood, in which the entire frakking forest snatches your pteranodon rider out of the air and smashes it into the dirt.
Then there's a specific treefolk, Crabapple Cohort, whose flavor text details it exacting its revenge upon people who picked its apples.
Liege of the Tangled, while not a walking tree himself, can turn all of your lands into a vengeful army that can curbstomp dragons in their wake. While he can turn any land into this, his name and color implies trees (the Tangled is the forest part of Mirrodin, covered in metal trees).
In Warhammer, the Wood Elves have a unit that's a giant tree monster that disguises itself as a beautiful elf, then eats anything evil that gets close. Hopefully, it will become a playable class one day in the MMORPG, because of Rule Of Cool.
Wood Elves also have treemen, which are pretty much the same thing as Tolkien's ents.
Warhammer 40000 plays around with this trope a bit; the Spiker (seed turns you into another one) and Brainleaf (spikes you and you turn into a zombie) are both horrifying, but non-sentient. This hasn't stopped at least one account in the lore of the Imperium seeding an Ork world with the things to keep the planet unusable.
There are several other deadly plants in the 40K galaxy, some typical carnivorous plants, others more unusual. The official website has a nice article about them, complete with rules to use them in games.
Dragon Dice has the Treefolk, an entire race/army of sentient, mobile trees and their Naiad and Dryad allies, animated by Mother Nature as a last ditch effort against the forces of Death.
Mage Knight miniatures has the Tanglewood Spirit, and later, the Oak Warrior and Oak Mage. If moving trees are scary, how about a tree that casts spells?
"Century Trees" in Deadlands: Lost Colony inhabit the Toxic Jungle on the planet Banshee. They're mobile, they're named for their height (over 100 feet tall!) instead of their age, and they don't like you.
Poisonwood trees in Talislanta. They don't attack, but don't touch these things if you value your life. Don't get shot with a poisonwood arrow either.
The Star Wars tabletop RPG introduced Darth Rivan, Sith Lord and sentient plant.
One DuckTales episode seems to turn this on its head. When Scrooge goes to an island to retrieve a lost play of the Shakespeare-analog of the world, he's attacked by animated trees, among other dangers inspired by Shakespeare plays. They're actors in tree suits, of course, pledged with defending the lost play, which is actually quite horrible.
Pokémon gives us Shiftry, Exeggutor, Tropius, Snover, and Abomasnow. When you have an entire type of Mon devoted to plants there's bound to be a lot of these. Interestingly enough, Sudowoodo looks like this, but is actually a tree mimic made of rock.
The Living Forest in Mortal Kombat had talking trees that were (falsely) rumored to be able to eat the players as a special fatality. Midway took care of this oversight in their later MK games.
The infamous Territorial Oaks, and their Palette Swap Hostile Elder Oaks, from EarthBound. These, by the way, explode when you kill them. At the level you first encounter them, you usually survive only if you can press buttons fast enough to exit battle before your health meter runs out.
And to make matters worse, they carry 1/128 items as well. Hope you enjoy pain, collectors!
There's also the Woodoh of the prequel, and the Tree and Tender Loving Tree of the sequel. Most of them explode as well.
Brigandine includes a man who's been sorta absorbed into the roots of a tree.
Whispy Woods from Kirby. Kirby Super Star introduces Twin Woods, which is basically two Whispys at once, and Kirby 64 adds mobile saplings named Whispy Woods Jr.
The Devouring Earth in City of Heroes have animated trees (Bladegrass, Razorvine, Deathblossom) and big animated mushrooms (Fungoids, Deathcaps) among their ranks.
La Pucelle had an overpowered demon tree for a boss. You can choose to go the easy way by going to a separate room to weaken it, or you can take the hard way and beat it down as it is. The hard way earns you a neat equipment at the end of the chapter.
One of the first major enemies in Golden Sun is King Tret, who normally isn't that hostile. Unfortunately, a Psynergy crystal impacted his trunk, giving him a vicious Split Personality that turns everyone within range into trees. Once that persona is under control (i.e. the heroes beat the sap out of it in a Boss Battle) and his injuries are healed, Tret gets his old self back.
Treants and the larger Ancients in War Craft III are guardians of nature and common allies of the Nightelves (the latter type serve as buildings and can be uprooted to fight or relocate the base. In World of Warcraft, Balance druids can summon treants to fight for them, while Restoration Druids can become treants without any offensive capacities but improved healing.
Treants are generic fighting machines in Warcraft, but in Defense Of The Ancients, there is a Treant Protector which is a hero with one of the best strength ratings in the game - a tree that not only attacks, but causes the woods around him to attack also.
Early in Final Fantasy Mystic Quest a boss is disguised as a tree. Later on in the game is the King of the Forest, a giant tree that can walk and talk. The King is good, in fact you must go inside him and destroy the monsters poisoning him.
Not to mention the various tree-shaped regular enemies. The first, Mad Plant, is fond of putting the heroes into full-nelson holds. Yes, really.
The enchanted dead trees near Draynor Manor in RuneScape, and also in the Haunted Woods area; both places are controlled (or were controlled, in the case of the former) by Vampyres. This may or may not be a coincidence.
The Holy Sapling boss of Magical Starsign is a gigantic tree monster that covers a good portion of both DS screens. Its main attacks are inhaling one party member (it has a face), gnawing on him or her, then spitting them out; and growing buds that bloom into flowers, then launching the petals at you. When you defeat it, it becomes a passive Man-Eating Flower.
About halfway through the game, Final Fantasy V's Exdeath, a powerful wizard who is the epitome of the Evil Overlord archetype, is revealed to be an evil tree. (More specifically, a tree that had had so many evil spirits sealed inside it that it became sentient—and evil—itself.)
Final Fantasy IX has the living avatar of the Iifa Tree, Soulcage, as a particularly nasty boss. Those who try to attempt the logically obvious "Kill It with Fire" will be unpleasantly surprised.
Not quite this, but Final Fantasy X has the Guado race, who are humanoid, but are molded in a way that makes them resemble trees (long arms, bushy hair, wide shoulders and thin hips, etc.). They retroactively resemble Spriggans.
They also live among giant trees.
Touhou has the Saigyou Ayakashi from Perfect Cherry Blossom, a demonic cherry tree. We never do actually get to see it in action because it is magically sealed by the body of its final boss, Yuyuko Saigyouji, and you spend the game preventing this seal from breaking; however, it is known to have killed many, many humans in the past.
Taken to ridiculous heights in the side-scrolling mecha game, Xardion. The first area of the last level involves bomb lobbing flying/hopping trees trying to kill you once you land as you make your way to the planet's core. This is obviously the beginning of the Mind Screw section.
Aggressive plants are a fairly common enemy in Guild Wars, ranging from small but annoying seed-things to full-blown mobile trees, often with spellcasting ability.
Corrupted trees in Okami pelt you with rotten fruit.
In Quest for Glory I, there is a Dryad who is very protective of the forest, and if you kill or threaten the local wildlife, she'll transform you into one of the local wildlife.
There's a miniboss in Wario World aptly called Tree Freak who is exactly one of these dangerous, semi sentient trees and is fought three times in one of the levels.
In a side story of Tsukihime, it is revealed that one of the 26 Dead Apostle Ancestors is a forest that absorbed the blood of an Ancestor that Arcueid killed. It was named after the former Ancestor and became known as the "Forest of Einnashe". The entire forest gained sentience and was known to wander around, swallowing entire towns in its wake and tempting people to enter with rumors of an immortality-giving fruit tree. Being an entire forest, not much could be done about it for all of 800 years. That is, until Satsujinki came along...problem solved.
In Illbleed, one of the boss monsters is a demonic tree that's taken over a lumber mill. Amusingly enough, when you first get to the boss it's undergoing repairs. Apparently, even in an Amusement Park of Doom attractions sometimes break down.
The Legend of Zelda: Ocarina of Time has you enter the Great Deku Tree as the first dungeon, who is the god of the Kokiri and a talking tree. He doesn't attack you, though the many Deku Babas and Scrubs will, presumably as a result of Ganondorf's death curse on him.
Wizardry VII: Crusaders of the Dark Savant had hostile trees as one of its less common enemy types, though they were largely unremarkable save for the spell-casting Halloweeches. One optional sequence in the game involves the party being assaulted by seemingly half the forest in several waves of attackers.
Kingdom Of Loathing: During October 2009, thanks to the weird in-game calendar, Arrrbor Day (an in-game holiday where pirates plant trees, hence the multiple r's) was just a few days away from the real-world Halloween. Thus "Arborween" was born: the Arrrboretum became the Shivering Timbers, with Halloween-themed trees (pumpkin trees, toilet-papered trees, the UnderworldTree, etc).
Sort of happens in Overlord; the Elven hero Oberon, consumed by Sloth, has merged with a giant tree, which animates in order to attack the... er... "hero".
The trees that inhabit Xen in the last levels of Half-Life, which attack anything that moves near it and, thanks to Good Bad Bugs, can be used to kill one of the ridiculously tough Gargantuas. It is speculated that the massive, impossible-to-kill Tentacles that are encountered in several portions of the game is its mature form.
The first Ape Escape had... er... vehicular trees in the forested levels that certain monkeys would operate, initiating a small boss fight. The Trees were rooted to the spot, and could be avoided by staying outside the range of its flailing branches.
Dragon Age has sylvans, which are trees possessed by demons and spirits. They tend to lie in wait until someone passes by them, at which point they move to attack. The vast majority are mad, violent creatures that can cause sharp branches to erupt through the ground and impale smaller beings. A rare few seem benevolent and don't mean any harm to others.
Age Of Wonders: Shadow Magic Elves got a Treeman. "Concealment" there means that if this wall-crushing behemoth stands in a forest, foes will not see it until it's one step away.
Mega Man 2: While Wood Man is a robot, he's certainly walking-tree-themed.
The fourth stage of Gun Nac was the Wood stage, so one of these naturally appeared as the boss.
Battle for Wesnoth brings us Woses, similar to Tolkien's Ents in personality, but more like Huorns in appearance.
ADOM has an entire level crammed with extremely tough walking trees. It's possible to get through the level slowly by waiting for them to move at random and fighting the few hostile ones... or by just smashing your way through it with brute force. Using fire attacks (which they are weak to) will make ALL the trees attack you.
Patapon 3 has Treants, who normally don't move, but, if you try to use fire weapons on it, it will wake up and start flinging fireballs at your army. And if it happens to be a rainy day, they will gain HUGE amounts of health back.
Throughout the Thief series, the Pagans often uses tree-like creatures to guard various locations.
In Dark Souls, Darkroot Garden has plenty of Demonic Foliage, and they sometimes like to hide in the ground to ambush you. There are also the sort-of mobile (they can rotate) Possessed Trees, but they're just their to be cut down to open up paths.
In Science Girls, most of the enemies look like plants, and there's a Treeman boss. Subverted in that they're not biologically plants.
In Minecraft, the Creepers are mottled green, have multiple legs but no arms, and according to Word Of God their hide feels like leaves. This makes them strongly resemble mobile, explosive plants.
Notch has said that, were they real, their texture would be "crunchy, like dried leaves", basically meaning they're supposed to be concussive walking topiaries.
In Mario Kart DS's Luigi's Mansion track, the forest contains some trees that scoot about. They like creeping into the way of racers (making them more like trolling trees) but are otherwise harmless.
There are a couple of different (but similar looking trees) in Shining The Holy Ark. You're walking long and suddenly the ground rumbles and a tree will grow in front of your face in the matter of seconds. It'll then proceed to attack you...for some reason.
The Dendroid Guards and Soldiers of Heroes of Might and Magic. Slow, but strong, and their attacks leave their victims rooted in place
Durkon of The Order of the Stick has obviously read too many books that contain them since he thinks every tree is dangerous. His fear is justified when he meets an evil druid with a small army of trees.
Not just him. His whole species hates trees: they launch WARS against them.
Priest of Thor: Nay, the enemy has been quiet ever since last year's Cabinet-Making Offensive.
It's considered a major crisis when a tree root manages to break through the walls of the underground homes.
Durkon: Just one tree has Colossal size, natural armor, damage reduction, hundreds o' hit points, and enough limbs ta make a dozen or more attacks per round.
In Kiss Wood the forst curses people so every plant in the forest starts going after them. It starts slowly but eventually giant roots will openly attack them.
Gunnerkrigg Court has "bound dogs", which are mobile trees whose roots are shaped into giant canines. They're said to be puppets or drones, not sentient creatures. There's also this more traditional tree-man.
In the NeopetsTale of Woe plot (a site event), Mr. Krawley enchants a group of trees in the Haunted Woods to attack Gilly as she attempts to pursue him. The trees also showed up as Battledome challengers during the plot.
Those horribly stupid trees in Aqua Teen Hunger Force, in that one episode that has Shake and Carl dump a vat of fryer oil in the woods. They actually took him to Wood Court. They also used Carl's skin as paper.
The Redwoods from Inhumanoids are sentient trees who give assistance to the heroes every so often.
Those spikey branch throwing trees in My Little Pony The Movie. Some viewers even found them scarier than the main villains.
Santa Claus is Coming to Town had the Winter Warlock trapped Santa and his penguin buddy using Killer Trees.
The "epipha-trees" that smack Homer around in a hallucination he has in The Simpsons Movie.
The actual series mentioned that trees cannot defend themselves, except for the Mexican Fighting Tree, of course.
In the Sealab 2021 episode "Isla de Chupacabra", one of these eats Stormy at the end of the episode. Considering the island also has the Chupacabras, the talking venomous hanging tree cobras, and presumably other horrors, it's an excellent example of a Death Island.
The Laughing Tree from Little Bear was a scary tree that grabbed people and wouldn't let them go unless they made him laugh.
In The Grim Adventures Of Billy And Mandy, the Taking Tree in Billy's yard. It steals items and will only give them back if they give it something better in return. It takes Billy and requires an extremely rare baseball card in return.
One villain from Krypto The Superdog brought every tree in the city to life, hoping they would follow this trope. The scheme fell apart for two reasons: one) the trees actually appreciate the humans that took care of them, and 2) they were trees. They don't like moving around much and prefer being rooted to the ground.
The Nightmare Before Christmas has a singing, walking tree with hanged skeletons dangling from it. It never actually attacks anyone, but most inhabitants of Halloween town are only meant to scare rather than harm.
The Sandbox Tree. It is a poisonous tree. Its bark is covered with spikes. It grows up to 100 feet. Its sap is highly poisonous and corrosive. Its fruits are rather large and explode when mature, catapulting its razor-sharp claw-like corrosive-poison-laced seeds in a 300 feet radius with a loud bang. So, essentially, this tree has a 100m wide Instant Death Radius. If it could walk, it would enslave mankind.
English folklore has the rhyme "Elm do brood, and oak do hate, but the willow walks when you travel late". Oak coppices were thought to be full of Oakmen, vengeful fairies who fed travellers poisonous fungi. Willows were thought to uproot themselves after dark, to walk behind travellers and mutter.
When the death rate in New Guinea is broken down by cause, tree-related accidents come at the top of the list.
This is mainly from people either climbing coconut trees to harvest the fruit and falling. However, a surprising number of injuries and deaths are from people napping or walking beneath coconut trees and having a ripe coconut fall on their heads. The trees can get up to 25 to 30 meters high, and the nuts can weigh up to two kilos. The impact force from a heavy coconut falling from one of the taller trees can easily crack a skull, and sometimes is fatal.
In a less tropical version of the coconut example, you have the sugar pine. It produces enormous pine cones known unsubtly as "widowmaker" cones. Stand beneath them at your own peril.
They don't need to explode to kill you. Several subspecies of eucalyptus also known has Gum have a tendency to shed potentially rather large branches and for some of the species that does this, the bigger branches tend to weigh in at a good 15 or so pounds so that landing on your head from 10 or so feet is a good way to meet your maker.