'non-mythical' is a Modern English compound formed by prefixing 'non-' to 'mythical', where 'non-' is a productive negating prefix and 'mythical' relates to 'myth'.

'mythical' ultimately comes from Greek 'mythos' (μῦθος) via Latin and Late Latin forms ('mythicus') into Middle English; 'non-' as a negative prefix comes through Old English and Medieval Latin usage and was attached to adjectives in Modern English to form negatives (e.g. 'non-mythical').

The elements originally meant 'not' (for 'non-') and 'pertaining to myth' (for 'mythical'); together they have been used in modern English to denote 'not pertaining to myth', i.e., factual or lacking legendary qualities.