The Wingless Fairy Series, Book 7: Rebecca and the Flying Horse Problem by Margaret Pearce

It should have been a peaceful night. Instead, something attacked Sally the horse under the full moon, leaving deep wounds and no tracks behind.

When Sally gives birth to a strange little filly named Saturday, Rebecca suspects something magical is at work. She is right. Saturday is no ordinary foal. She talks. She climbs trees with hidden claws. And hidden beneath her oversized shoulders are wings waiting to unfurl.

But with wonder comes danger. A black flying horse with glowing red eyes is hunting the foals, and it returns with every full moon. Time is running out. The babies cannot breathe the air of the human world for long.

With help from her wise owl companion, loyal house goblins, and a friendly river troll, Rebecca must protect her growing magical family and face a threat that strikes from the sky without warning.