A successfully conjured Patronus can take two forms: non-corporeal or corporeal, and both types vary greatly in their appearances and strength.

Non-Corporeal Patronus

A non-corporeal Patronus is one which does not resemble any living
creature and has few distinguishing features (if any). Non-corporeal
Patronuses resemble a burst of vapour or smoke without any clearly
defined form shooting from the tip of the wand. While they may be
partially effective at halting dementors (but not repelling them),
non-corporeal Patronuses are not "fully-fledged" Patronuses, and are
regarded as a more primitive or weaker version of the true Charm.

Corporeal Patronus

A corporeal Patronus is one that is fully-formed, taking the shape of
bright-white, translucent animal. The specific animal forms that
corporeal Patronuses take vary from person to person and reflect each
individual's personality, so each Patronus is unique to each individual witch or wizard.