The name boa comes from the word bos, which means 'cows.' This name was used because of the odd feeding patterns of boas,  which attach their mouths to the udders of cows and suckles their milk.  Either the boa kills after its finished,  or the boa will drink so much milk that the cow dies.  Thus, the name boa comes from its devastating effect on oxen and cattle. 
Later bestiaries declare the boa to be a serpent found in Italy, including the twelfth-century Aberdeen Bestiary and  Isidore of Seville's Etymologies from the seventh century.  However, in the earlier Historia Naturalis, boas are described as snakes in India.  Some sources describe boas as a cross between a serpent and a large dragon. 
Boas have been known to follow flocks of gazelles and herds of cattle,  pursuing them so they can feed from their milk.  Despite their size, boas are so swift that they can snatch birds flying overhead and swallow them down in one gulp. 
The History of Boas
The earliest surviving written reference to boas can be found in Pliny the Elder's Historia Naturalis, written in 77 AD. 
General Regulus killed a boa during the First Punic War, which occurred from 264 to 241 BC over the western part of the Mediterranean Sea, including land battles in Sicily and in coastal Africa (Ancient Carthage).
According to legend, the boa was 120 feet (36.6 meters) long and lived near the Bagradasa River (also referenced as the river Rhyndacus in Pontus). General Regulus attacked the boa as if he were storming a town: 
Generall under the Romanes, during the warres against the Carthaginians, assailed a Serpent neere the river Bagrada, which caried in length 120 foot: and before he could conquer him, was driven to discharge upon him arrowes, quarrels, stones, bullets, and such like shot, out of brakes, slings, and other engines of artillerie, as if he had given the assault to some strong towne of warre.
-- Pliny's Historia Naturalis 
The General took the skin and jawbone of the boa to prove his feat, and the trophies remained in a Roman temple until the Numantine War,  which started in 154 BC.
Another legendary report of a boa occurred during the reign of Emperor Claudius, between 10 BC and 54 AD. After being slain on Vatican Hill, the body of a whole child was found inside its stomach. 
Later descriptions and illustrations showed boas as large snakes with visible ears, smaller wings, and sometimes legs. 
- The word 'boa' comes from the word bos, meaning 'cow.' 
- The fantastical boas can be found in India  and in Italy. 
- The name comes from the creature's odd feeding habits, as it would stalk herds of cattle and suck their milk dry. 
- Pliny's Natural History Book VIII, Chapter XIIII
- Rose 53
- The Medieval Bestiary - Boa
- The Aberdeen Bestiary: Folio 69r Translation and Transcription
- Allardice 40
For more information on footnotes and references, please see the bibliography.