Definition: Let a unicorn be a magical horse with a horn protruding from the forehead. Let a unicornex be an existent unicorn. No non-existing thing is a unicornex. So, it’s true by obversion, some existing thing is a unicornex.
More about the term: Atheist philosopher of religion William Rowe argued that Anselm’s ontological argument (probably) begs the question. He used the example of a ‘unicorn’ and a ‘unicornex’ to display this charge of circular reasoning. This is what Rowe believes Anselm is doing by just defining God into existence.