What Is An Illustration Of A Nonexistent Thing

What is an illustration of a nonexistent thing?

Exist any fictitious things, i. Zeus, Pegasus, Sherlock Holmes, Vulcan (the fictitious planet proposed by 19th-century astronomer Le Verrier), the perpetual motion machine, the golden mountain, the fountain of youth, the round square, etc. Something being nonexistent is the same as saying it doesn’t exist, so what is another word for nonexistent? Either the thing didn’t exist at all or it did exist, but it’s no longer there, depending on the context.

What is the single word for “doesn’t exist”?