The Last Unicorn (1982) - IMDb

In an enchanted forest, a unicorn learns she is the last of her kind; a butterfly reveals that a demonic animal called the Red Bull herded her kind to the ends of the earth. Venturing into unfamiliar territory beyond the safety of her home, the Unicorn journeys to find them and bring them back.

When the Unicorn nears the seaside castle of King Haggard, supposed keeper of the Red Bull, she encounters the animal, which turns out to be a monstrous fire elemental. At the last moment before her capture, Schmendrick's unpredictable magic transforms her into a human woman with white knee-length hair. In this guise the Red Bull is uninterested in her and departs. The Unicorn suffers tremendous shock at the feeling of mortality in her body. While Molly wraps the Unicorn's human form in a blanket, Schmendrick states that the magic chose the form and not him.

Schmendrick, Molly Grue and the now-human Unicorn proceed to King Haggard's castle. Haggard is at first unwelcoming, and Schmendrick introduces the Unicorn as his niece, Lady Amalthea. Schmendrick requests that the three of them stay there as members of Haggard's court, only to be told that the only occupants of the castle are Haggard, his adopted son Prince Lír and four ancient men-at-arms. Haggard consents to lodging the trio, replacing his more competent on-call wizard with Schmendrick, and setting Molly Grue to work in his scullery.

Amalthea begins to forget her identity and her reasons for coming to the castle, and falls in love with Prince Lír. Caught in her newfound emotions, she struggles with thoughts of abandoning her quest for the sake of mortal love. Haggard confronts Amalthea in private conversation, hinting at the location of the unicorns, yet from the waning magic in her eyes, he has doubts regarding his previous suspicions that she is more than she seems. Molly learns the location of the Red Bull's lair from a talking cat.

On the beach, the Unicorn magically revives Lír before departing for her forest. Schmendrick assures Lír that he has gained much by winning the love of a unicorn, even if he is now alone. The Unicorn briefly returns to say goodbye to Schmendrick, who laments that he has done her wrong by burdening her with regret and the taint of mortality. She disagrees and thanks him for having helped to restore unicorns to the world; though she is the only unicorn to feel regret, she is also the only unicorn to know love. Schmendrick and Molly watch the Unicorn depart for her home in the woods.