The weasel is a mammal belonging to the Mustelidae family. A weasel was one possible corporeal form of the Patronus Charm. Arthur Weasley's Patronus took the form of a weasel.

What Harry Potter character has a weasel Patronus?

Arthur Weasley's is a weasel.

Arthur conjures his Patronus in "Deathly Hallows" in order to send a message to Harry, Ron, and Hermione.

What does it mean if your Patronus is a weasel?

Those who cast a weasel are as playful and mischievous as anyone would expect. Many of these witches and wizards are spontaneous and quick to make decisions without taking much thought on the consequences. ... Those who cast a weasel patronus are dangerous, relentless, and very intelligent.