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 animagi take the same form as their Patronus.

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 Transfiguration 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, but the full extent and quality of it is unknown, as normal animals minds are not as complex.

A Homorphus Charm can be used to force an Animagus out of their animal form. It can be used non-verbal to any witch or 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 human, Animagi, while in their animal forms, are safe to be in the company of werewolves. Also, a werewolf is forced to undergo transformation, while an 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 also available to the public. The penalty for failing to register as an Animagus in a sentence in Nurmengard for six months. Due to the complexity of the magic involved in becoming an Animagus, they have become very rare. There was a total of seven registered Animagi in the twentieth century. It is also partially because of this complexity, in addition to the danger involved, that Animagi are required to register themselves.