Extract from ‘Charms of Defence and Deterrence’ by Catullus Spangle
The Patronus represents that which is hidden, unknown but necessary within the personality.
For it is evident 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.
Herein lies 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 recognise.
Admittedly, some Patronuses do, in rare cases, resemble animals for which the caster feels an affinity. 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