This is a double definition - the clue gives you two different meanings of the same word, and you need to find the word that satisfies both definitions.
Look for a word that means both 'horse' (specifically a female horse) and also relates to something that 'appears on the moon' - think about lunar geography and the names given to features on the moon's surface.
Answer: MARE | Double definition - a mare is a female horse, and a mare is also one of the dark, flat areas on the moon's surface (from Latin meaning 'sea', though they contain no water).