The Unicorn Pool by Tiddy Ogg

The story is an amalgam of a Theodore Sturgeon short story, "The Silken Swift", with the characters in Thomas Hardy's "The Woodlanders" - well, someone had to give poor Marty a happy ending if only in a thing like this, and the novel "A Kid For Two Farthings" by Wolf Mankiewicz, about a boy who buys a young goat with a single deformed horn and believes it to be a unicorn.