It’s a children’s book, I think middle grade level?  Not sure though, but I don’t remember it having pictures.  A girl wants a horse for her birthday, but her parents get her a model horse instead.  Girl is disappointed (I would be too!).  She climbs a tree with the model and puts it on a branch, but is mad at it and flicks it down to the ground.  It comes to life (full sized) and talks.  She rides it around.  I don’t remember how it ends.  I probably read it in the early 90’s.