Two entries with the name Porrex.
Porrex
According to Geoffrey of Monmouth, a king of Britain in the third or second century BC.
He was the son of King Coill and the father of King Cherin.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Porrex
An early and legendary prince of Britain who, according to Geoffrey of Monmouth, was a descendant of Brutus. Porrex ruled in the sixth or fifth century BC.
The son of Gorboduc and Judon (Iudon), he had a younger brother by the name of Ferrex, with whom he quarrelled over the right of succession. Porrex plotted to ambush his brother, but he fled to Gaul, whence he returned with a Gaulish army, but was defeated and killed.
As Ferrex had been her favourite, Judon was driven insane by grief, and, while Porrex slept, she hacked him to pieces. As neither Porrex nor Ferrex had left an heir, the line of Brutus died out when Gorboduc died. Afterwards, the kingdom of Britain became fragmented until Dunwallo Molmutius reunited it.
Another tradition says that after his death, Prydein came from Cornwall and conquered Britain, thus leading to the association of Prydein as the eponym of Britain.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138