When I was younger, I saw a movie about a girl who moved into this new house or with a relative, I believe it was with her uncle. She is bullied by her cousin. The girl takes the cousin's horse for a ride but ends up crashing into a giant thing of hay. Here I believe she meets the leprechaun person for the first time. They talk and he leaves before the uncle and/or cousin comes and finds their horse in the field all tacked up under the tree. They soon realize that the dark brown horse's knee is bleeding.
The girl gets in trouble and is grounded and/or given work to do around the stable. Later that night she hears something and the leprechaun has weird smoke coming out of the window and she goes there and sees that the thing ate the apple pie that was made for them the next day. The uncle walks in and gets angry at her because he assumes she ate the pie.
At some point she is in the woods and comes into contact with a white horse. She trains on him for a while and cares for it. Then she asks her uncle if she can compete in the cross country or jumping show against her cousin. At first he said no but he later agreed. Before the big competition, the cousin cuts a stirrup so that she will have an advantage while racing her.
In the end the stirrup breaks off but the girl beats her cousin. The cousin gets very angry and throws a tantrum. The uncle then sees the behavior and tells her to cool down. She keeps talking so he locks her in with her horse in the mobile stable.
That night she has to let her horse go back into the wild because she discovers that it is a unicorn and that its unicorn horn has been cut off. So she lets it go and that's all I remember.