A person can technically patent something that is fictitious if it meets the criteria of being new, useful, and non-obvious, typically applying to inventions or concepts rather than physical entities.
However, the mere existence of a patent does not confirm that the patented item exists in reality. Patents protect the intellectual property and the inventive process rather than the physical existence of an object.
And the following cannot be patented:
Laws of nature
Abstract ideas
Methods of medical treatment
Products of nature
Natural phenomena
Abstract concepts
Mental processes
Mathematical formulas
Abstract ideas that are not tied to a specific machine or process.