Treants (i know i've heard it somewhere and i don't know where) are rare mobs that (rarely) spawn in Forest, Jungle, Taiga, Mountain, Tundra, Extreme Hills and Ocean Biomes (basically all biomes with trees). They look just like a regular tree depending on its biome (Tundra and Taiga will have snow on top of them) and once the player hits one of the wood blocks (melee only), the tree will gain legs and walk towards the player, damaging him if he comes close. When he's under half health, he starts throwing out little, red blocks (apples) that also damage the player if they touch him, but these have very low health. Once the tree dies, it drops 6 Wood, 1-3 Heartwood and a random Wood-tier weapon.