What I would do first is think about who you are. (If you would like some 'words' for you, look up horoscopes and find your Sun and Moon sign. But, I wouldn't recommend that.) What are your traits? How do you go about things? Are you an escapist? Are you calculating? Or just go? Think hard

Now think about how Rowling put characters and their animals together. It's pretty simple:

. Harry's is a Stag - kind and graceful, but with powerful antlers to protect himself and others.
. Ron's is a Terrier.
. Hermione's is an otter- Always gathering and collecting things, like info. Otters always collect things, like clams and such and use different tools like rocks to open them.
. Lilly Evan's & Severus Snape's is a Doe- Graceful, delicate and as Dumbledore said, "unfailable kind" and with a heart of gold.
. Luna's is a Rabbit - Kind and Wise, always digging and always out-smarting predators.
. Aberforth's is a Goat - Stubborn, rude and Stagnant.
. Sirius Black's is a Dog- Smart, Loyal and will protect you till the end.

When I chose mine I was taking these into consideration and how Rowling put everyone's.

When I chose mine. I chose Cheetah. I wanted a Stag before, but that's the wrong gender and If I were an Animagus, I would change genders, so yea. Well anyways, I chose Cheetah. (Research them if u want). When stag didn't work I went back to square one. What are my favorite animals? I love horses, probably cause I have horses. Cross that off. I love Deer. Crossed off(stag issue). I went back to my deeply loved animal, Cheetah. I have always had a strange liking to cheetahs. That's something I could never figure out, since I was 5! I try to hide it, but can't help it. I love cheetahs!
We tend to like the things we relate to, so I thought about it hard and thought. "Yea cheetahs fit me". Cheetahs calculate every movement and won't go for the hunt unless they are sure its right. That's me. Cheetahs are cats yes, but differ from other big cats greatly - they cannot roar, they have 'grayhound type bodies', they are fast, their claws to not go in, etc. They also are escapists and have many enemies, such as Lions, Other cats, Baboons, Hogs,etc. I am an escapist and have a tendency to runaway from things I don't wanna do, either by fear or stubbornness. My presence it not as bold as a Lion, but I do have a presence. Cheetahs are always moving and they migrate. I am always changing and like change, because I get bored easily and stuff. When cheetahs do something they wanna do, it's perfect and full force. Cheetahs tend to live alone and choose to be, like me, but when they have a friend it's for life and will protect them with all their might, like me. They also are good at getting around and figuring things out.

As you see, you could relate me to a cheetah in many ways as Rowling's character's related to their animal.

What might you relate to? All about Animagus and Patronus is pasted bellow.

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What is a Animagus and Patronus??? (THIS IS ALL COPIED FROM: http://harrypotter.wikia.com/wiki)

Animagus

""Please, sir. An Animagus is a wizard who elects to turn into an animal. A werewolf has no choice in the matter." -Hermione Granger to Severus Snape

An Animagus (pl. Animagi) is a witch or wizard who can morph him or herself into an animal at will. It is a learned, rather than hereditary skill, unlike those of a Metamorphmagus. Information on Animagi is taught to Hogwarts students during their 3rd year.

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, with or 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. An animagus in animal form is also not restrained by the lifespan limit of the creature that they otherwise transform into, seen in the case of Wormtail, who stayed in the form of a rat for at least 12 years, remaining alive despite the fact that rodents have a short life cycle.

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 primary reason for registration is to ensure the Animagi do not abuse their abilities, as the Ministry is able to keep track of them. One example is because Sirius Black is an unregistered Animagus, he was able to use his form to escape from Azkaban, as the Ministry did not take this into account when imprisoning him.

This is all from:

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Patronus Charm:

"A Patronus is a kind of positive force, and for the wizard who can conjure one, it works something like a shield, with the Dementor feeding on it, rather than him. In order for it to work, you need to think of a memory. Not just any memory, a very happy memory, a very powerful memory Allow it to fill you up... lose yourself in it... then speak the incantation "Expecto Patronum"."
Remus Lupin teaching Harry Potter the Patronus Charm

The Patronus Charm is a charm that evokes a partially-tangible, positive energy force known as a Patronus (pl. Patronuses[2]). Patronuses are also called spirit guardians[3][1] though this may only refer to corporeal Patronuses. It is primarily designed for defence against otherwise unbeatable Dark creatures like Dementors and Lethifolds, though there are other uses.

To successfully cast the Patronus Charm one must muster a happy memory (the happier the more powerful and tangible the Patronus will be) and incant "Expecto Patronum". It is unknown if there are any other elements requires in casting, such as physical gestures (aside from drawing and pointing the wand forward prior to or during incantation).

The Patronus Charm is widely regarded as advanced magic (so much so that its not even taught in Charms on the Hogwarts curriculum). It is a very complex charm and many qualified wizards and witches have trouble with it. In fact, Harry Potter is one of the youngest known wizards able to cast a Patronus; he was taught how to do so in 1994 at the age of thirteen by Remus Lupin[4]. In a Dumbledore's Army lesson, Harry taught the members how to use the charm. Some were even successful in casting a corporeal form, though Harry said this might be because there was no Dementor to make them frightened[5]. However, some members were later shown to be able to cast corporeal Patronuses in the presence of Dementors[6].

A corporeal Patronus is one that is completely and fully formed and takes the shape of a bright-white, translucent animal. The particular animal the corporeal Patronus takes the form of, like Animagus forms, has been said to reflect the personality of the caster. They are, however, subject to change if the caster goes through an emotional upheaval of some sort. For example, Nymphadora Tonks's Patronus changed from its previous form to that of a four-legged creature which others guessed was a werewolf because then it would reflect her love of Remus Lupin, himself a werewolf[5].
Another example is Severus Snape, whose Patronus was that of a doe, the same as Lily Evans, though this may have been the case all along rather than a change from its original form, as Snape was in love with Lily for most of his life. If a wizard is an Animagus and can summon a corporeal Patronus, the two may take the same form, as was the case for Minerva McGonagall and James Potter[6]. Most Patronuses take the form of an ordinary animal, but magical creatures, such as Albus Dumbledore's phoenix Patronus, are not uncommon. The corporeal patronus is also better at repelling Dementors.

There are two known uses for the Patronus charm. The first and main use of the Patronus is to drive off certain Dark creatures such as Dementors and Lethifolds, and is the only known charm that will work against them. As Dementors feed on happy memories to render humans to drown in sorrow and sadness, the Patronus acts as a shield of sorts, of which the Dementor would try to feed off of instead - the Patronus is comprised exclusively of positive feelings and cannot comprehend negative emotion, so the Dementor's influence cannot harm it.

The other use was devised by Albus Dumbledore, who invented a way of using the Patronus (corporeal and non) as a means of communication. They can deliver messages, speaking with the voice of the caster. This source of communication is believed to be exclusive to the members of the Order of the Phoenix. Using the Patronus for communication offers great advantages in security, as Patronuses identify the caster and are Dark-proof.[5] A good example of this was when Minerva McGonagall used her feline Patronus to call for help to the other heads of houses in 1998 when she found out Harry Potter had returned to Hogwarts in search of Rowena Ravenclaw's Diadem. Another is when Severus Snape used his Patronus to lead Harry Potter to a pool in the Forest of Dean in 1997, where he had laid Godric Gryffindor's Sword for Harry to retrieve.

This is all from: http://harrypotter.wikia.com/wiki/Patronus

HOPE THIS HELPS! XD

Oh yes, you can "nickname" them too!! Sirius Black's was a dog, Padfoot. James Potter's was a stag, Prongs, Pettigrew was a Rat, wormtail. Me, my cheetah is Lightfoot.