About Cry at Midnight by Mavis Gulliver:
On their first night in Aunt Aggie's cottage on the Hebridean Isle of Tiree, Merryn and Hamish McQueen begin an exciting, but terrifying adventure. Finding a carved box containing a necklace of sea-beans and hag-stones, Merryn discovers that she possesses The Gift. Inherited from her great-great-great-great-grandmother, Merryn McQueen, she is able to connect with magical beings, both benevolent and malevolent.
With her brother's help, she embarks on a mission to save a horse called Kester. Trapped in a fencepost by a witch, Kester can be released by Merryn between midnight and sunrise. Riding through successive nights, she and Hamish are hindered by Aunt Aggie's strict rules, and beset by the witch's increasingly powerful spells. Selkies and Fairy Folk pledge their help, but not until seemingly impossible challenges have been met. Kester and the children travel to Highland locations; the famous Ringing Stone and the beach of Traigh nan Gilean, where we will finally discover whether Kester will be freed and where his true identity will be revealed.
Mavis Gulliver lives on the Isle of Islay from where she travels extensively throughout the Hebrides.