[1] Adler reports a similar ('cf.') entry in the Ambrosian Lexicon. As noted in LSJ and illustrated by the quotation given here, lower-case si/nis can indeed mean any plunderer/robber. However, the Sinis, the Pine-bender (LSJ s.v. II), was a notorious mythological robber operating near Corinth, killed by Theseus: see under theta 368 (with the spelling Sinnis).
[2] Greek Anthology 7.202.3-4 (Anyte), about a rooster, quoted already at rho 167; cf. Gow and Page (vol. I, 38), (vol. II, 97), and another extract from this epigram at epsilon 2933. Anyte does not identify the marauding animal, but see rho 167 n.2 for possibilities.