I'm looking for a movie I saw a long time ago when I was a kid, it was a brother and a sister and they we're on some kind of quest, and along the way they encountered all these strange mystical creatures. It was a cartoon but extremely realistic, some of it almost looked like live action. There was a red monster that could breathe fire it was the main antagonist of the story. I swear this movie exists I just can't remember the title for the life of me.

The main antagonist of the movie The Last Unicorn is The Red Bull. A huge bull monster, bright red, that breathes fire, but there is no brother and sister in this film.