Mammoth Uncensored Word List:
25 words ending with an, 15 letters long

acanthocephalan agribusinessman antiegalitarian antilibertarian antitrinitarian appendicularian biostatistician crossopterygian edriophthalmian lactovegetarian malacopterygian nonarchosaurian nonmetropolitan nontotalitarian northcountryman ornithodelphian parliamentarian plainclothesman platitudinarian polyphloesboean postmillenarian psychohistorian psychometrician quinquagenarian tetrahydrofuran