It's about a princess or the like, who her mother (a queen I think) left at the door of a farmhouse owned by a childless couple one night. When she got a little older, this Black Horse came to her and become a sort of her guardian and friend.

I totally remember this cartoon, I used to watch it with my sisters, try this:
Talk About Cartoons: Wildfire Intro & Theme Song (http://talkaboutcartoons.blogspot.com/2008/05/wildfire-intro-theme-song.html)