Rachael King 
Grimmelings [EPUB ebook] 

The same evening Josh Underhill went missing, the black horse appeared on the hill above the house. Ella knows that words are powerful. So she should have known better than to utter a wish and a curse on the same day. Who is Gus, the boy with the impish grin, who seems to appear in answer to her wish? And what does the black horse want? When Ella finds that her grandmothers warnings of creatures that dwell in the lake are more than just stories, she and her pony Magpie are drawn into a dangerous, life-saving mission.