I think this is the first Girls Underground example I’ve covered where the girl is (originally) an animal. Sometimes Girls Underground get transformed (like in The Cat Returns) but they usually start as humans (or, anthropomorphic at least).
The Last Unicorn is a classic 80’s fantasy animated film based on a novel by Peter S. Beagle (but this post just refers to the film version). One day the unicorn learns she is the last of her kind, which basically fits the “orphan” archetype. Initiated on her quest by a rambling butterfly, she sets off to find out what happened to all the other unicorns. The story is that an evil creature called the Red Bull has driven them to the edge of the world.
She is soon captured by a witch, but in captivity makes friends with her first companion, a mediocre wizard named Schmendrick. When they escape together, they encounter bandits in the forest and acquire another companion, Molly. They travel to the castle of King Haggard, who controls the vicious Red Bull. But when the unicorn is directly threatened by the bull, Schmendrick tries to save her and instead turns her into a human – the bull leaves, but the unicorn quickly loses her magic and starts forgetting who she is.
The companions all stay for awhile in the castle. The unicorn, thinking herself a mortal woman, falls in love with the King’s adopted son, and gets distracted from her mission of finding her own kind. But after going through a magical portal, all is revealed to the prince, and the unicorn is returned to her original form. When the Red Bull kills the prince, the unicorn finds the strength to confront it, and in turn frees the other unicorns who had been trapped in the sea. The castle crumbles, the king is killed, but the unicorn revives her prince, although she can no longer make a life with him.