I am looking for at least two books. Each book contained a collection of scary stories. I used to borrow them from the library where they were in the children section (up to age 12, but I'm unsure about the specific age category).
One story was about a boy who changed into a bat at night. I remember vividly he ate moths and would find pieces of them (between his teeth or on his pillow?) when he woke up.
One of the other stories was about a girl who saw the ghost of a horse in a meadow in the morning, possibly walking along a living horse. I believe the ghost horse was either white or grey. It might have been hers once. The clue was that she found out it was a ghost and that the horse died of eating from a yew tree which grew at the edge of the meadow.
The books must have been published before 1990. I remember them as hardcover books with at least one book having a dark red cover. The other was possibly blue. Illustrations were few, black and white and not in-your-face scary.
I read them in Dutch and I have no idea what language the books were originally in.