Latin -monium comes from PIE *-monyom (from *-mō). daemonium comes from δαιμόνιον/daimónion, which comes from δαιμόνῐος/daimónios, which comes from δαίμων/daimōn + -ῐος/-ios, and -ῐος comes from PIE *-yós. lemonium comes from λειμώνιον/leimṓnion, which comes from λειμώνῐος/leimṓnios (probably also from PIE *-yós).
-monyom and -yós are different suffixes. -mō is for agent nouns. -yós creates adjectives from noun stems. AutisticCatnip (talk) 20:09, 18 June 2017 (UTC)
Same elements. *-monyom, substantive use of *-monyos, i.e. *-mō + *-yós, > -mōnium. *-mō > -μων + *-yós > -ιος = -μὀνιος, then -μόνιον. Of note is -ō-, which appears also in some Ancient Greek forms as λειμὠνιος. -GuitarDudeness (talk) 22:03, 18 June 2017 (UTC)