The headword, presumably extracted from the quotation given, is a neuter noun in the nominative (and vocative and accusative) singular; see LSJ s.v. and cf. ps.-Zonaras 1468. Adler also cites Lexicon Ambrosianum 514.
[1] Named for 'a group of Illyrian peoples who had absorbed Celtic influences to varying degrees' (OCD(4) s.v.), the Roman province of Pannonia (founded in 9 CE) lay south and west of the middle Danube region (Barrington Atlas map 20 grids C3-E4), roughly in what now occupies parts of Slovenia, Croatia, Bosnia, and Serbia; cf. pi 206, pi 879, and eta 277. In her critical apparatus, Adler notes an alternative spelling in mss GSM, *pano/nnioi, for the name of the ethnic group, i.e. the entity relevant to the present entry; but for the regular spelling cf. the toponym under alpha 527 and nu 561.
[2] Polybius fr. 64 (Büttner-Wobst). The fragment is unplaced, and its attribution to Polybius is uncertain (Büttner-Wobst, p. 523; Walbank, p. 748). The compound e)ch|rh/kesan is indeed unlikely (Walbank, ibid.), and, as Adler notes, Hultsch suggested e)chrtu/kesan (they had fitted up) as an alternative (Hultsch, p. 555).