Otter

An otter is a semi-aquatic mammal known to the Muggle world. Within the Wizarding world, its primary and most significant role is as a form for the Patronus Charm. It is famously the corporeal Patronus of Hermione Granger, and its characteristics are often seen as a reflection of her personality.

The otter's main relevance in the magical community is as a possible form for a witch or wizard's Patronus. This form is most prominently associated with Hermione Granger.

• Hermione Granger's Patronus: Hermione first successfully casts a corporeal Patronus during a Dumbledore's Army meeting in Harry Potter and the Order of the Phoenix. It takes the form of a “silvery otter” that is described as bright, sleek, and playful, tumbling and gambolling around her.
• Messenger and Guide: In Harry Potter and the Deathly Hallows, Ron Weasley's Deluminator leads him back to Harry and Hermione. Shortly after, a silvery-white otter Patronus appears in the Forest of Dean. Harry Potter recognizes it as Hermione's and follows it to a frozen pool where the Sword of Gryffindor lies hidden. The Patronus was actually cast by Severus Snape, who used the same form as Lily Potter's doe to match Harry's expectations, but Harry Potter initially mistook it for Hermione's.
• Combat: Hermione's otter Patronus is used in combat during the Battle of Hogwarts, helping to drive away Dementors alongside the Patronuses of Luna Lovegood and Seamus Finnigan.

Otters are often associated with intelligence, curiosity, and playfulness. These traits strongly align with the character of Hermione Granger. Her deep intellect and thirst for knowledge mirror the creature's cleverness, while the Patronus's joyful and gambolling nature reflects a lighter, more spirited side of her personality that is not always at the forefront. The connection between a witch or wizard and their Patronus form is known to be deep and revealing of their “secret self,” making the otter a fitting animal spirit for Hermione.

• J.K. Rowling has stated that the otter is her favourite animal. (J.K. Rowling interview)
• The author chose an otter for Hermione's Patronus because she has a strong personal affinity for the character and sees a great deal of herself in her. (J.K. Rowling interview)
• The film adaptations of Harry Potter and the Order of the Phoenix and Harry Potter and the Deathly Hallows – Part 1 visually depict Hermione's otter Patronus as an ethereal, bright silver creature that swims gracefully through the air. (film)
• The otter is one of the possible forms a user can be assigned after completing the official Patronus quiz on the Wizarding World website. (Pottermore)