Treant (CR 8)

Huge Plant
Alignment: Usually neutral good
Initiative: -1 (Dex); Senses: low-light vision, Listen +8, and Spot +8
Languages: Treant, Common, Sylvan

AC: 20 (-2 size, -1 Dex, +13 natural), touch 7, flat-footed 20
Hit Dice: 7d8+35 (66 hp); DR: 10/slashing
Fort +10, Ref +1, Will +6
Speed: 30 ft.
Space: 15 ft./15 ft.
Base Attack +5; Grapple +22
Attack: Slam +12 melee
Full Attack: 2 slams +12 melee
Damage: Slam 2d6+9
Special Attacks/Actions: Animate trees, trample 2d6+13, double damage against objects
Abilities: Str 29, Dex 8, Con 21, Int 12, Wis 15, Cha 12
Special Qualities: Plant, fire vulnerability
Feats: Improved Sunder; Iron Will; Power Attack
Skills: Diplomacy +3, Hide -9*, Intimidate +6, Knowledge (nature) +6, Listen +8, Sense Motive +8, Spot +8, and Survival +8 (+10 aboveground)
Advancement: 8-16 HD (Huge); 17-21 HD (Gargantuan)
Climate/Terrain: Temperate forest
Organization: Solitary or grove (4-7)
Treasure: Standard

Source: Monster Manual

Treants combine features of trees and humans. They are peaceful by nature but deadly when angered. They hate evil and the unrestrained use of fire, considering themselves guardians of the trees.

Treants are almost indistinguishable from trees. Their skin is thick and brown, with a bark-like texture. Their arms are gnarled like branches, and their legs fit together when closed to look like the trunk of a tree. Above the eyes and along the head are dozens of smaller branches from which hang great leaves. In winter the leaves of a treant change color, but they rarely fall out.

Treants speak their own language, plus Common and Sylvan. Most also can manage a smattering of just about all other humanoid tongues - at least enough to say "Get out of my trees!"