The Katobleps may have been derived from an embellished traveller's account of the African gnu.
Aelian, On Animals 7. 6 (trans. Scholfield) (Greek natural history C2nd A.D.) :
"Libya [i.e. Africa] is the parent of a great number and a great variety of wild animals, and moreover it seems that the same country produces the animal called the Katobleps (Catoblepas, the Down-Looking). In appearance it is about he size of a bull, but it has a more grim expression, for its eyebrows are high and shaggy, and they eyes beneath are not large like those of oxen but narrower and bloodshot. And they do not look straight ahead but down on to the ground: that is why it is called ‘down-looking’. And a mane that begins on the crown of its head and resembles horsehair, falls over its forehead covering its face, which makes it more terrifying when one meets it. And it feeds upon poisonous roots. When it glares like a bull it immediately shudders and raises its mane, and when this has risen erect and the lips about its mouth are bared, it emits from its throat pungent and foul-smelling breath, so that the whole air overhead is infected, and any animals that approach and inhale it are grievously afflicted, lose their voice, and are seized with fatal convulsions. This beast is conscious of its power; and other animals know it too and flee from it as far away as they can."
Pliny the Elder, Natural History 8. 77 (trans. Rackham) (Roman encyclopedia C1st A.D.) :
"In Western Aethiopia (Ethiopia [i.e. West Africa] there is a spring, the Nigris, which most people have supposed to be the source of the Nile . . . In its neighbourhood there is an animal called the Catoblepas, in other respects of moderate size and inactive with the rest of its limbs, only with a very heavy head which it carries with difficulty--it is always hanging down to the ground; otherwise it is deadly to the human race, as all who see its eyes expire immediately."
- Aelian, On Animals - Greek Natural History C2nd - C3rd A.D.
- Pliny the Elder, Natural History - Latin Natural History C1st A,D,