Foal In The Fog

Whilst pony trekking with her cousin in Devon, Mandy gets separated and lost in the fog. She comes across a lone skewbald foal, who immediately disappears back into the mist. Local girl, Abby, is haunted by the death of Curiosity, her own skewbald foal, who had lost his life in the fog. Might it be Curiosity that Mandy has seen? If so, why has he come back?