It is evident from old woodcuts and scrolls that the Patronus Charm has been used since ancient times. It therefore is unknown who created it, or when it was first invented.The charm also has a long association with those fighting for lofty or
noble causes (those able to produce corporeal Patronuses were often
elected to high office within the Wizengamot and Ministry of Magic).

While there is a widespread and justified belief that a wizard who is
not pure of heart cannot produce a successful Patronus (the most famous
example of the spell backfiring is that of the Dark wizard Raczidian,
who was devoured by maggots), a rare few witches and wizards of
questionable morals have succeeded in producing the Charm.  It may be that a true and confident belief in
the rightness of one’s actions can supply the necessary happiness. 
However, most such men and women, who become desensitized to the effects
of the Dark creatures with whom they may ally themselves, regard the
Patronus as an unnecessary spell to have in their arsenal

No
reliable system for predicting the form of an individual’s Patronus has
ever been found, although the great eighteenth-century researcher of
Charms, Professor Catullus Spangle, set forth certain principles that
are widely accepted as true.

The Patronus, asserted Spangle,
represents that which is hidden, unknown but necessary within the
personality.  ‘For it is evident,’ he writes, in his masterwork ‘Charms
of Defence and Deterrence’:

‘… that a human confronted with
inhuman evil, such as the Dementor, must draw upon resources he or she
may never have needed, and the Patronus is the awakened secret self that
lies dormant until needed, but which must now be brought to light...’

Here,
says Spangle, is the explanation for the appearance of Patronuses in
forms that their casters might not expect, for which they have never
felt a particular affinity, or (in rare cases) even recognize.

Spangle
is interesting on the subject of those unusual witches and wizards who
produce a Patronus that takes the form of their favorite animal.

‘It
is my firm belief that such a Patronus is an indicator of obsession or
eccentricity.  Here is a wizard who may not be able to hide their
essential self in common life, who may, indeed, parade tendencies that
others might prefer to conceal.  Whatever the form of their Patronus,
you would be well-advised to show respect, and occasionally caution,
towards a witch or wizard who produces the Patronus of their choice.’

The
form of a Patronus may change during the course of a witch or wizard’s
life. Instances have been known of the form of the Patronus transforming
due to bereavement, falling in love or profound shifts in a person’s
character.  Some witches and wizards may be unable to produce a Patronus at all
until they have undergone some kind of psychic shock.

It is
usual, but not inevitable, for a Patronus to take the form of an animal
commonly found in the caster’s native country.  Given their long
affinity with humans it is perhaps unsurprising that among the most
common Patronuses (although it must be remembered that any corporeal
Patronus is highly unusual) are dogs, cats and horses.  However, every
Patronus is as unique as its creator and even identical twins have been
known to produce very different Patronuses.

Extinct Patronuses
are very rare but not unknown.  Strangely, given their long connection
with wizardkind, owl Patronuses are unusual.  Most uncommon of all
possibly Patronuses are magical creatures such as dragons, Thestrals and
phoenixes.  Never forget, though, that one of the most famous
Patronuses of all time was a lowly mouse, which belonged to a legendary
young wizard called Illyius, who used it to hold off an attack from an
army of Dementors single-handedly.  While a rare and magical Patronus
undoubtedly reflects an unusual personality, it does not follow that it
is more powerful, or will enjoy greater success at defending its caster.