The Wolf Block is a mob-spawn system which can be crafted by the player and never naturally found.
( CB - Cobblestone, B - Bone, WH - Wolf Head )
A Wolf Block spawns a hostile wolf that will attack the mob / player that passes it, it will not spawn a wolf to hurt the player who placed it though. If a wolf passes the block the Wolf Block will spawn a wolf to attack the player who placed it (this does not work with tamed wolves). If a player that passes the block tames the hostile wolf, the tamed wolf's first aim is to hurt the player that placed the block.