Professor Umbridge’s Patronus takes the form of a Persian cat in Harry Potter and the Order of the Phoenix.
Dolores Umbridge produces the Patronus while occupying Hogwarts and using the magical creatures to intimidate the centaurs in the Forbidden Forest. The fluffy cat form is deliberately ironic because Umbridge decorates her office with plates showing kittens, even as her behaviour becomes increasingly authoritarian.
A Patronus is a protective magical manifestation. Harry’s Patronus is a stag, Hermione’s is an otter, and Snape’s is a doe, so these forms are easy to mix up when recalling the films.
Umbridge’s Patronus is specifically shown as a cat associated with her character’s Persian-cat decorations. It is not a sign of her being a cat Animagus; a Patronus and an Animagus transformation are different kinds of magic.