A Patronus is a magical guardian which a wizard may summon by using the spell Expecto Patronum. It is a complicated spell called the "Patronus Charm." A Patronus can be used to defend one self from dark creatures like Dementors or to deliver messages. The latter was a use of Patronuses which was discovered by Albus Dumbledore.
 Summoning a Patronus
Summoning a Patronus is known to be very hard; many adult wizards are unable to produce one. One thing is successfully using the spell; another thing is to make the Patronus take the solid shape of an animal. When Harry Potter cast a succesfully stag-shaped Patronus at the age of 13, he became one of the youngest wizards to ever do so. He was taught to cast a Patronus by Remus Lupin, who he had seen casting one at a Dementor.
To summon a Patronus - even a small foggy one - the spellcaster has to think of an extremely happy moment, preferably the happiest they have. This process makes it hard for many wizards to summon their Patronus, as bad memories or ideas can influence the happy memory or idea. Slytherin's Locket had such an influence on Harry, Ron and Hermione. They were also unable to summon proper Patronuses after Fred Weasley died during the Battle of Hogwarts.
Successful Patronuses take the shape of different animals for different people. The first animal seen as a Patronus was Harry's stag. His father had a stag too, suggesting they had quite similar personality. This is a case similar to what form an Animagus would take. As a fact, both James Potter and Minerva McGonagall were Animaguses (stag and cat respectively), and both had a Patronus resembling their Animagus form. The shape of a wizard's Patronus can also change if they experience a huge emotional change, as was the case with Nymphadora Tonks.
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