Treeman
"We go, we go, we go to war, to hew the stone and break the door; For bole and bough are burning now, the furnace roars - we go to war!"
- – The Ents' Marching Song, LoTR
As the name suggests, a Treeman is a towering plant-monster that resembles a tree in basic stature and appearance, but which is capable of getting up and walking around like a man. Naturally associated with dense woodlands, they tend to be averse to fire and look down on people who carelessly clear trees or start fires, not being above beating the shit out of anyone who they feel doesn't respect the forest.
Treemen are often associated with dryads, though exactly how so varies from game to game. In one setting, they may be male and female of the same species. In others, they may respect each other on a professional level, as dryads tend to be focused more on specific trees whilst treemen guard the forests as a whole.
The most famous example of treemen in fantasy comes from Tolkien's trilogy, The Lord of the Rings. Known as the Ents, they are an ancient race of humanid plants created in the dawn of the world and responsible for creating, tending and guarding the forests of Middle-Earth. Tolkien's ents are literally tree-men, being the males of the species; the females, known as the Entwives, went sailing across the sea thousands of years ago and haven't been seen since. Legend has it that this idea stems entirely from the fact that Tolkien saw a production of Macbeth and was disappointed that the prophecy of Birnam Wood coming to Dunsinane wasn't accomplished by the trees coming to life and walking there.
Dungeons & Dragons
Naturally, Dungeons & Dragons promptly followed in Tolkien's footsteps, since the game basically began as a Lord of the Rings army game turned RPG, avoiding a lawsuit from the Tolkien estate by calling its treemen... wait for it... "Treants". Yes, it is one of the stupidest names you could imagine, but it passed in court, and it's stuck around for a good five editions so far.
Warhammer Fantasy
Treemen are the most powerful and fearsome of the forest spirits that are allied with the Wood Elves.
They are spirits in tree form that were sprout from the power of Oak of Ages, and it happened way before Chaos invaded the world. Since old one was hinted to be the one that planted the Oak of Ages' seeds, meaning these treemans are some kind of alien tree creature from outer space. Being a tree spirits also making them tougher than any mortal beings (except flaming attack) like the demons of chaos, allowing them to deal magic attacks in both melee and range. It was also for sharing too much similarity like a chaos demons that it unnerves the later enough to not mess with them.
The Treeman were peaceful back when Chaos hasn't invaded, and was even willing to communicate with outsiders like Elves from Ulthuan. The Treeman had taught the Elves on the secret of tree caring, which the Elves took its knowledge to their heart and taught them their languages in return.
After the chaos invasion, many Treeman had grown vengeful that they had adapt their body for the purpose of combat. They also changed their foreign policy to a more xenophobic approach, even begin to holding grudges without copying a Dwarf. Elves were still accepted into the forest, but that doesn't mean the Treemans like it that much.
Speaking of Dwarf, those beardy axe fuckers came to Athel Loren after the War of the Beard to find some Elves to kill in. Unfortunately for the Treemans, they are weak during winters that they were made easy prey for the angry beardlings. The Elves' heroic action against the Dwarfs is what allows them to stay in the forest.
Durthu would be the representative of the toughest and oldest treemen of all, who is constantly hunted by his need for vengeance and hatred against his old foe that wronged him, especially the dwarves, but he blamed himself the most. He also has a giant orange sword forged by Daith(a blind elven smith who supposedly not only older than Orion and Ariel themselves, but also forged Aenarion’s Dragon Armour).
In Total War: Warhammer, tree spirits like Durthu and Treeman speak in some kind of alienish language in their dialogue, which is unknown if the lore specify what language they speak, but probably Elvish due to Durthu's name is Oakheart in Elvish.