3 His brother Morgan, pricked with proud disdain,
brother > cousin, kinsman Morgan > (Son of Goneril) disdain > indignation
4 To have a peer in part of sovereignty,
peer > rival part > [the] function, office, business; hence: discharge sovereignty > royal authority
5 And, kindling coals of cruel enmity, 6 Raised war, and him in battle overthrew: 7 Whence, as he to those woody hills did fly
fly > flee
8 (Which hight of him Glamorgan), there him slew:
hight of > were named after Glamorgan > [Glen of Morgan]
9 Then did he reign alone, when he none equal knew.
equal > [equal to him]