'non-mammal' is a compound of the negative prefix 'non-' and 'mammal'. 'non-' comes from Latin 'non', meaning 'not'. 'mammal' originates from New Latin 'Mammalia', ultimately from Latin 'mamma' meaning 'breast'.

'mammal' was formed from New Latin 'Mammalia' (from Latin 'mamma') and entered modern English as 'mammal' in the 18th–19th century; the productive English prefix 'non-' has long been used to form compounds meaning 'not X', producing forms like 'non-mammal' (or 'nonmammal').

Initially the root referred to animals characterized by 'mamma' (breasts) and 'mammal' meant animals with mammary glands; 'non-mammal' has consistently meant 'not a mammal' and retains that straightforward negative meaning.