The headword is a feminine abstract noun in the nominative (and vocative) singular; see LSJ s.v. (The quotation which follows has the genitive plural of the cognate substantive mo/narxos, monarch; see LSJ s.v.)
[1] The glossing noun is much rarer than the headword. Besides here, it is attested only in poems of Gregory of Nazianzus (with an Ionic ending monokrati/h) and Theodore Prodromos (later than the Suda).
[2] Polybius 11.13.8 (web address 1), where 'foreigners' effectively means mercenaries (cf. generally xi 33, xi 39, xi 44). The context is the third battle of Mantinea (OCD4 s.v. and s.v. 'Mantinea, battles of,' mu 165, and mu 163; Barrington Atlas map 58 grid C2) in 207 BCE; see Walbank, p. 284.
[3] (See generally LSJ s.v.) Likewise in the Lexica Segueriana, De syntacticis s.v.