Some possible monsters to consider:
Basilisk 0r Regulus (“Little King”) from Cyrene, Libya
A small snake “not longer than twelve fingers” with a crown shaped crest on its head. At times, the basilisk is seen with the head of a cockatrice due to its odd birthing ritual involving a toad and cockatrice.
Eats almost anything and attacks by bite or gaze; its bite or gaze is extremely lethal.
Sources: Pliny the Elder’s Natural History
Yale or Eale (“To move back”) Ethiopia
Looks like an antelope or goat-like creature that is the size of a hippopotamus, with an elephant’s tail, usually black or tawny in color, with the jaws of a boar and movable horns.
Eats people and large animals and rams its prey with its horns and tusks.
Sources: Pliny the Elder’s Natural History.
Cacus ("The Evil One”) A giant who breathes fire and smoke. He is the son of Vulcan originally from Rome on the Aventine Hill.
Eats human flesh, but not their heads. He nails the heads of his victims decoratively outside his cave. He attacks and kills its enemies and prey by breathing fire and smoke onto them.
While Cacus has not been seen since Hercules apparently strangled him to death.
Sources: Virgil, Aeneid, Ovid, Fasti, Propertius, Elegies.
Amphisbaena (“Mother of Ants”) from Libyan Desert sprouting from the blood of Medusa’s head, and later seen by Cato’s army.
It is a two headed serpent, whose tail has the second head; however this “serpent” is about the size of a long worm. The addition of wings and chicken feet was reported by some sightings. It has a poisonous bite.
Sources: Pliny the Elder’s Natural History, Aelian, On Animals, Diodorus Siculus, The Library of History.
Manticore ("Man-Eater") from Persia, Parthia with the body of a red lion, a human head, with a trumpet-like voice. Sometimes it is seen with horns or wings and eats people and large animals. Attacks with is tail which has been reported in the form of a dragon or scorpion and it can shoot poisonous spines that paralyse and kill its victims.
Sources: Ctesias, Indica, Pausanias, Guide to Greece, Aelian, On Animals, Philostratus, Life of Apollonius of Tyana, Pliny the Elder, Natural History, Eusebius, Against Hierocles, Photius, Myriobiblon.