A choose your own adventure where a girl and her brother? Have to save a mythical animal.

I need help finding a choose your own adventure type story. I remember that it starts with a young girl and boy going to the circus. Once there they find out the circus master is evil and keeping some mythical animal unethically. The girl and the brother have to try and save the creature. That's all I remember. I wanna say I read it when I was 10 years old (2003) or somewhere close to that. But the book could definitely be older than that.