Level 6 : Final Exam : Knowledge of Animagus

Animagis are rare because they must possess an animal’s brain. If they did not they would not be able to remember why they became that animal and hence forget what it was that they were doing in the first part. In order to be an animagus you must be able to not only turn into an animal but be able to control your animal form and able to know what it was you were doing and why you turned into an animal. If you could not do this there is no reason why you should turn into an animal to just forget what it was you were doing. This is why it is different when another wizard turns you into an animal because instead of being able to control yourself you will start to act like the creature you were turned into.
It is the animal that chooses the wizard would be an accurate saying because the animal that they change into is not what the wizard picks but is picked from their personality. So if you were wise you might turn into an owl, brave a lion you might be and so on. Once you have turned into the animal that fits your personality then you might be able to understand other different animals too which is probably be the part of your brain that is animal like which allows you to do this.
Not only would you be able to communicate with other animals but you would be able to “talk” to other Animagis too. Since you both share that unique part of your brain that allows you to talk to animals you will be able to understand one another. There is a setback though; you won’t be able to talk to any humans. You might be able to show them messages in the dirt or sand but if you were to try to talk to them they would not understand a word that you were saying. They would just hear you barking or making whatever noise your animagus form makes.
When turning into an animagus there will be a distinguishing mark that indicates to the Ministry that you are not in fact that creature but a wizard taking the form of that creature. If you wore glasses there might be some markings around your eyes that would show this, or any other mark that sets you apart from everyone else.
The Ministry had set many laws to make sure that the few Animagis do not abuse this gift they were given. One is that they must all be register to insure that the Ministry knows all of whom can do this rare form of transfiguration. Another is that they must change into their animal form away from any muggles that might be nearby. Those are a few of the laws that the Ministry had set for them.
There are many advantages to becoming an Animagis. One is that if you were able to turn into a bird you could then just fly from one place to another without using floo powder, apperation or brooms. You could just turn into the bird, spread your wings and fly away. Another good thing is that if you where to turn into a dog or cat you would be able to run faster and cover ragged terrain quicker. The disadvantages to this is that if you where to turn into a magical animal like a dragon then you would not be able to get near nay muggles because you would just frighten them. Another disadvantage is that you could frighten wizards which might use some unwanted magic on you ending up hurting either yourself, the other wizard or both of you.


souces
http://www.hp-lexicon.org/magic/animagi.html

Animagi are the ultimate fusion of animal and man, and the highest level of Transfiguration. It is a joining so perfect, so exact that the Animagus retains his usual human brain functions, while amassing all the basic instincts and skills of the animal it has transformed into. Though we know how to become Animagi and what the training encompasses, nobody has quite figured out exactly WHY the process will transform a wizard so beautifully into an animal. That is why it is under intensive study at the Department of Mysteries.

According to extensive research done by the Department of Mysteries, most witches and wizards use only 12% of their brains' total capacity. An Animagus, on the other hand, will use 15% of their brain's total capacity. Further research led the Unspeakables to conclude that all the Animagi had higher IQ's (Intelligence Quota) and MQ's (Magic Quota) than regular people. They were able to become Animagi only after they had learned to utilize more of their brain.

Several studies showed that even after long years of training and self-examination, witches and wizards with lower IQ's and MQ's were unable to transform, or needed a special low stress environment and higher personal concentration to transform.

On the other hand, it has been recorded that some witches and wizards were able to transform after a few paltry years, with seemingly no effort whatsoever each individual time, once they had learned the basics. It would seem that some witches and wizards are just BORN with the ability to naturally utilize more of their brains. That is one reason why Animagi are so rare. Though technically speaking, anyone could become an Animagus, one can understand that an average witch or wizard would not resolve to train for 4-10 years or more just to possess this ability.

Another reason for the rarity of Animagi is this: there can be devastating side effects of Animagi training. So many things can go wrong that usually witches or wizards planning to undergo training will be supervised by the Ministry. It would be quite a mess indeed if the wizard was unable to retain his human intelligence, and ended up roaming the streets with a tiger's head and appetite. There is no going back once the process is mangled.

That would be why all Animagi must register with the Improper Use of Magic Office, once when they want to start undergoing training, and again if they have reached the level of Animagus. This register is known to the public and keeps note of what kind of animal the Animagus transforms into, and what their distinguishing markings are. Understandably, there are severe penalties for failing or refusing to register with the Improper Use of Magic Office.

Transforming, the first few times, is slightly painful and uncomfortable. Reported side effects include dizziness, tingling in limbs, temporary loss of vision, vomiting and sore muscles. Every time thereafter, however, the side effects lessen and eventually disappear entirely. A seasoned Animagus, one who has transformed many times, will report pleasurable feelings when transforming. There is a sudden fluidity in their muscles, a grace in their movements, a clarity in their thoughts.

Even the most talented Animagi cannot choose the animal they transform into. It will be whichever animal their personality most resembles. Animagi's markings are determined by what the witch or wizard looks like when in human form. If the wizard is balding, their fur will be sparse. If they are missing an eye, their Animgus, too, will be missing an eye. Clothing and accessories, such as robes, glasses, watches, etc. will be absorbed into the form and will reappear when the wizard transforms back into a human. But witches and wizards who always wear their accessories, to the extent that the glasses or watch has become a part of who they are, then the accessories will appear on the Animagus form as markings.

Animagi can communicate with other animals, because any word they utter will automatically turn into a bark, growl, howl, etc. However, most smaller animals are afraid of bigger Animagi, as the animals can sense the superior power and intelligence emanating from the Animagi. But the smaller Animagi need to beware their natural prey, because a leopard will not stop for conversation before it begins to stalk a deer.

Because Animagi still retain their human intelligence, they can figure out ways to communicate with humans. They are incapable of human speech, but they utilize body language and signs to make humans understand. It is no surprise, then, that Animagi communicate best with other Animagi. They possess the same level of intelligence and read each other's body language. They can communicate with both, the animal and human parts of their fellow Animagi.

Sources: hp-lexicon.org and my own overactive imagination.