The user can turn matter/others into any animal they desire, effects can be permanent or temporary depending on what level of power the user is. The user can reverse the effect if they so choose to. It's usually done as a curse.
After Haru entered in the Cat Kingdom (The Cat Returns) she slowly began turning into a cat.
As a goddess level sorceress, Circe (DC Comics) could to turn objects and people alike into various sorts of animals.
Principal Torres (Every Witch Way) turns Lily Archer into a frog.
Irene Belserion (Fairy Tail) can turn other people into animals, such as when she transformed Princess Hisui into a mouse.
Diego Brando's Stand, Scary Monsters (JoJo's Bizarre Adventure Part VII: Steel Ball Run) turns its victims into dinosaurs.
The Transformation Spell (Little Witch Academia) can be used to convert others into anything they desire, usually animals.
Loco (Marchen Awakens Romance) can use her Darkness ÄRM, Animal Chain to turn a person into any animal.
Babbo is turned into a powerful cat guardian when Ginta (Marchen Awakens Romance) activates Version 6: Puss in Boots.
SMILEs (One Piece)
Dr. Facilier (The Princess and the Frog)
The Cranky Wizard (Samurai Jack) unleashes a transformative blast that turns his victims into chickens.
SCP-2746 - ████ is dead (SCP Foundation) is a portal between an underground tunnel in Las Vegas and an alternate dimension (SCP-2746-1) that transforms humans who travel through it into animals.
SCP-4670 - Pigs in Blankets (SCP Foundation) is a former serial killer who can turn humans into pigs, an ability she used on several police officers in order to butcher them, cook their carcasses, and then serve them at her BBQ restaurant.
The Animalizer (The Thundermans) was invented by Dr. Colosso which can turn people to animals and vise-verse.
Wonder Woman is turned into a pig - Justice League Unlimited