Animagus

An Animagus (pl. Animagi) is a witch or wizard who can morph him or herself into a specific animal at will. It is a learned, rather than hereditary, skill, unlike those of a Metamorphmagus. All known animagi take the same form as their Patronus. However, if their Patronus changes, their Animagus will always be what it was orginally.

Acquiring the Skill
Only very powerful and skilled wizards are able to become Animagi. The process of becoming an Animagus is long and arduous, and has the potential to backfire and cause the transformation to go horribly wrong. Once the initial training is over, an Animagus can change at will at any time, without a wand.

Animagi can only take on the form of one specific animal. This animal form is not chosen by the wizard, but determined by their personality and inner traits. Thus, one's animagus form is a reflection of one's inner nature. It is unknown whether Animagi can take the form of a more "magical" creature.

Each Animagus bears an identifying mark on their animal form that is caused by something on their human body. This may be a physical trait like dental structure, or an aquired trait such as glasses.If an Animagus undergoes a major physical change, such as the loss of a limb, their animal form will also reflect that.Also, an animagus in animal form will retain the lifespan of an ordinary witch or wizard.

In addition, an Animagus will appear to lose their clothing upon morphing. The clothing will reappear upon reestablishing the human form.

Transfiguration vs. Animagus Transformation
The difference between mere Transfiguration and the Animagus Transformation ability is that an Animagus can change into an animal whenever they want, without a wand or an incantation. Being an Animagus is an ability, and Transfiguring requires a spell. An Animagus still thinks as a human does when they are in their animal form. However, an Animagus's feelings are not as complex when they are in their animal form.

An Animagus also has the ability to communicate with normal animals. There seems to be a clear understanding of want and need in this communication ability.

A Homorphous Charm can force an Animagus out of their Animagus form unless if countered by a very powerful wizard.

It is also important to note that a werewolf is fundamentally different from an Animagus. Lycanthropy is caused by a blood infection transmitted through the bite of a werewolf, whereas the Animagus skill is learned. The werewolves cannot keep their minds after a transformation (unless they take the Wolfsbane Potion), nor can they control when they wanted to transform, while Animagi can do both. However, as werewolves only harm humans, Animagi, while in their animal forms, are safe to be in the company of werewolves. Also, a werewolf is forced to undergo transformation, while Animagi do it at will.

Wizarding Law
Every Animagus is required to register with the Ministry of Magic at the Improper Use of Magic Office. This registration includes revealing one's animal form and one's distinguishing markings. The Register is available to the public.

The penalty for failing to register as an Animagus is a sentence in Nurmengard for 3 years.

Due to the complexity of the magic involved in becoming an Animagus, they have become very rare. There is a total of 11 registered Animagi in history, all still living. It is also partially because of this complexity, in addition to the danger involved, that Animagi are required to register themselves.