Sea Horses: Guardian of the Stone by Louise Cooper
Tamzin Weston unwittingly releases the evil spirit of the Grey Horse, who has been trapped inside a small statue for hundreds of years. Her mistake threatens to bring perpetual storms and danger. Only Tamzin can put things right and bring balance back to nature. To do this she must find and enlist the spirit of the Blue Horse, a force for good, who has battled and overcome the Grey Horse once before.