Are you referring to The Legend of Rah and the Muggles? That was found to be bullshit the second people read the book. It’s an extremely weird story about nuclear fallout, talking animals, and a shit ton of meandering filler. The only similarity was use of the word “muggle”, which doesn’t even mean the same thing in both stories. JK may be a TERF now, but she is not a plagiarist.