The second element here, petehno/s, is an epic and poetic variant of the first, peteino/s; other variants are pethno/s and pthno/s (see LSJ s.v.).
If either peteino/n (well-attested in the Septuagint and elsewhere) or petehno/n -- masculine accusative singulars or neuter nominative/accusative singulars -- are quoted from somewhere, their source is unidentifiable. Note, however, that Hesychius has an entry for the genitive plural petehnw=n, glossed with peteinw=n. (It is quoted from Homer, Odyssey 13.87; see the scholia there.)