Trying to remember a particular children's fantasy book

[BearAusten]

I am trying to remember the title and author of a book that I must have read in the 80s. It was a children's fantasy book. There was a ladybird in it that turned black. I think it was corrupted/turned evil. There was also an evil horse/stallion who changed from black to white, and thus good. This has been irritating me for years. Not much to go on.

Hmm, don't know the book, but might be worth posting on:
Stump the Bookseller

It's an American site, but they are fairly broad-ranging, and easier to use than Abe Books.

Thank you. Will try that.