I'm trying to find the title of this old cartoon movie I remember from when I was just a little kid. I don't remember it too well so my details might be a little off. I'm pretty sure it was a fantasy/adventure cartoon with a boy and a girl as main characters with some helpful creatures (some kind of animal I think, I forget).
I remember this one scene where they used some sort of gem as a key to open a door out of nowhere and ended up falling into a fast moving underground stream or something. I also remember this other scene (I think it was near the end) where the boy is leading a horse (or donkey or something) though a door using a carrot on a stick.
It was a long time ago and I might have some details mixed up.

I finally found it.
It was Fluppy Dogs.