Moonsilver - Kathleen Duey

Heart has no family. Found abandoned as a baby, she now lives a hard and lonely life in the dusty village of Ash Grove. Then one day she finds a scarred, skinny horse in the forest. Against the wishes of her mean-spirited guardian, Simon, Heart adopts the mare and names her Avamir.
When she realises Avamir is in foal, Heart is thrilled. And so too is Simon - now he has two horses he can sell to the slaughterhouse instead of one. But when the colt is born, Heart finds him weak and with a strange bump on his forehead. A bump that begins to crack open, to reveal a silver point beneath. . .
Heart realises that the horses are actually unicorns, magical creatures from ancient legend. But why are they here? And how is their destiny linked with Heart's own?
Being creatures of legends, Heart is afraid that if the unicorns aren't lead away from Ash Grove they will surely be captured and killed by the superstitious villagers. Can she find a way to protect them, and the only love she has ever known?