I’ve tried to find the original source for this illusion but cannot. It’s similar to what are called "topsy-turvy" optical illusions, where an image turned upside-down reveals another image. It’s never been called the "Anurequs optical illusion" before, except by me, because I wanted the appearance of the horse – if you’ve never seen this illusion before – to be a surprise, and you were less likely to be surprised if the title of this post was "The frog to horse optical illusion".
"Anura" is the scientific order of amphibians that includes frogs, and
"Equus" is the genus of mammals that includes horses, thus the oh so clever I have to explain it "Anurequs."