The Red Bull came for a unicorn, so she had to become something else.
4 seconds sound clip from the The Last Unicorn movie soundboard.
You can hear this line at 00:47:43.390 in the Blu-ray version of the movie.
Quote context
[...]
- What have you done? What have you done?
- Doubtless you are wondering how I plan to return her to her proper shape. Wonder not.
- I didn't know you meant to turn her into a human girl!
- The Red Bull came for a unicorn, so she had to become something else.
- The magic chose the shape, not I. I am a bearer! I am a dwelling! I am a messenger!
- You are an idiot! Do you hear me? You lost her! You trapped her in a human body! She'll go mad!
- I can change her back. Don't worry about it. I can change her back.
[...]