imaginary creature, 1926 (A.A. Milne), from a child's pronunciation of elephant.
name of an imaginary creature or thing, 1858, American English, fanciful formation.
1824, "one of an imaginary corps of mounted sailors," hence "a person out of his element and unfit for his place" [Century Dictionary], from horse (n.) + marine (n.). However by 1878 the term was being used in fact in reference to cavalrymen pressed into marine service or seamen mounted as an improvised shore defense.
1680s, imaginary rat-like after-birth believed to be gotten by Dutch women by sitting over stoves, 1680s.