Basic Trope: A character is transformed into a form far less useful than his/her default one.
Straight: During her encounter with Rowena, the hermit swamp's witch, Alice is transformed into a little rabbit by one of her spells.
Rowena, tired of the arrogant and decadent conduct of the citizens that live on the realm adjacent to her swamp, one day decides to pay them a visit and turns everyone into humble forest's critters like mice, squirrels, canaries and gophers.
Rowena turns Alice into a microbe.
Rowena transformed Alice into a humble rabbit in order to give a lesson. (Maybe she was being an arrogant jerk or cruel to the little fellas) She expects that she might learn something after walking a mile on the shoes... Erh, I mean, bare paws of a little beast.
Alice is under a timed spell, having been turned into a bird by Rowena to help her infiltrate the enemy camp. Nobody will notice a bird in a forest, and once she's past the guards, the spell will wear off and she'll turn back to human.
Rowena has been contracted by one of Alice's adversaries to capture her alive. And since she's a capable warrior, (or even magic user on her own right) it would be easier to transport her and keep her under control if she's in a less threatening form.
Zig Zagged: The hex casted on Alice keeps transforming her back and forth between rabbit and human at totally random intervals. Needless to say, this turns into an extremely annoying (and even dangerous) situation because she ends up transforming into a little critter when she is about to deliver a finishing upon a dangerous opponent, and her lack of "magical clothes" means whenever she changes backs into human, she has to sneak around while in the buff, looking for something to cover herself, in order to save her modesty.
Enforced: The actress portraying Alice in Chronicles of Troperia as to take a time off the series. So, the writers decide to turn her character into a cat to justify her absence without ditching the character on some convoluted way. They plan to transform her back once she returns.
Invoked: Rowena has always been an Actual Pacifist since her witch apprentice days, therefor incinerating / freezing / vaporizing her adversaries is an unacceptable option to her. So, she specialized herself in transformation spells in order to deal with foes to reckless to treat with diplomacy.
Played For Drama: Alice forceful transformation couldn't have arrived at a worst time, because it was on her wedding's day. Now her absence has been attributed to last-second-regret, with the groom heartbroken and devastated by it. She, as a rabbit, tries to explain him what happened, but neither he nor anyone can understand her noises and treat her as just an annoying pest. And the worst part is that Rowena seems to have just vanished from the face of Earth, and without a way to communicate herself with humans, there's a great chance that Alice might remain stuck as a little critter for the rest of her life!