Genitive singulars of the noun dai/mwn (spirit, god, demon); conventional -- i.e. trisyllabic -- in the gloss, but quadrisyllabic (by diaeresis) in the headword, which must be quoted from somewhere.
Plato, Cratylus 398B, etymologizes dai/mwn as from dah/mwn; perhaps this led some later poet to treat dai/monos as having four syllables.