Shapeshifter

Shapeshifting - The supernatural ability to alter and change the physical form of one's self.

Shapeshifting is the ability of werewolves, hybrids, and immortals, to change one's shape from human to either wolf or everything, respectively. To werewolves, the process is activated involuntarily every full moon while others, like hybrids, it can be controlled at will. Immortals can change their shape/ copy someone else shape.

Werewolves
During a full moon, a werewolf will unwillingly transform from their human form into their wolf form. The transformation involves the breaking of every bone in the werewolf's body and especially the first transformation can be a slow and very painful process, in time the transformation gets faster and less painful. The transformation, however, can be controlled with only four known ways: pregnancy, hybridization, "evolving" and through the use of magic, via moonlight rings. A pregnant werewolf will not turn for nine months as the transformation would kill the baby, while werewolf-vampire hybrids, evolved werewolves and werewolves wearing moonlight rings can control their transformations at will, regardless of the full moon.

Hybrids
Werewolf-vampire hybrids and Klaus, the Original hybrid, keep all their werewolf abilities but unlike normal werewolves, have also the ability to control their werewolf transformation at will, even during the full moon. Hybrids do not seem to be bound by any former rules concerning their transformations as they can both choose not to transform during the full moon but also keep their wolf forms unusually long. Upon breaking the Hybrid Curse, Elijah informed Klaus that he remained a werewolf for two days after his first transformation.

Most hybrids seem to avoid full transformation due to the pain it causes, preferring partial transformations, manifesting some features of their werewolf forms.

Immortals
Immortals have the ability to change their shape by copying one's of other humans or animals. They can change their shape whenever they want.