Benwig appears only in the Welsh prose tale Culhwch and Olwen, the earliest extended Arthurian story, preserved in the Red Book of Hergest and the White Book of Rhydderch. In this tale, Twrch Trwyth is an Irish king named Taredd, transformed by God's punishment into a monstrous boar with venom-dripping bristles and a comb, scissors, and razor lodged between his ears. He is accompanied by seven offspring, themselves transformed men who act as his warriors and bodyguard. Six of these piglets are named in the text: Grugyn Silver Bristle, Llwydawg the Killer, Twrch Llawin, Gwys, Banw, and Benwig. The seventh remains unnamed, a shadow among shadows.

The hunt for Twrch Trwyth dominates the latter half of Culhwch and Olwen and ranks as the most elaborate chase in medieval Welsh literature. Arthur and his host sail to Ireland and find the boar and his seven sons in their lair. Menw son of Teirgwaedd takes bird-form to spy on them, but when he tries to snatch one of the implements from Twrch Trwyth's head, the boar shakes himself and scatters venom, wounding the scout. The chase then begins in earnest, sweeping across Britain like a storm. The boar and his brood are driven from Ireland to Porth Clais in Dyfed, through Carmarthenshire and the Llwchwr valley, across the Preseli Mountains and down toward the Severn estuary.

At every turn the piglets defend their father with a fury that matches any knight's. They turn on Arthur's men and slaughter hounds and warriors alike, buying time for Twrch Trwyth to bolt away. The casualties mount with each stand: knights fall, dogs are torn apart, and the ground is littered with the dead before the boar moves on. The piglets are not mere animals but transformed warriors, retaining the cunning and courage of the kings and champions they had once been. They fight as a unit, Grugyn and Llwydawg leading the charge while the smaller piglets harry the flanks.

The first named piglet to fall is Twrch Llawin, slain early in the chase. Then Gwys is killed at Mynydd Amanw. The pursuit continues to Dyffryn Amanw, the Amman Valley, where Arthur's men finally close the net. It is here that Benwig and its brother Banw are slain together, the two little boars falling side by side in the valley that still bears their father's name. The text records their deaths in a single line, spare and final: there were slain Banw and Benwig. None of the pigs went with Twrch Trwyth from that place alive except Grugyn Silver Bristle and Llwydawg the Killer, who would follow their father to Cornwall and to the sea beyond.

Benwig's legacy is inseparable from that of Banw, its fellow victim at Dyffryn Amanw. The name Dyffryn Amanw, the Amman Valley, carries an onomastic link with banw, the Welsh word for pigling, and both little boars are remembered in the landscape where they fell. Modern Ammanford marks the place with sculptures of a boar and his piglets, built from stainless steel and bronze. Benwig may have been the least famous of the seven, the piglet whose name appears second in the death notice and nowhere else, but its death at Dyffryn Amanw is one of the fixed points in the geography of the greatest hunt in Welsh literature. The companion who is never named alone is sometimes the one who makes the story possible.