therefor > for that [for riding such a steed]
5 He had not trained been in chivalry.
chivalry > cavalry, horsemanship; knightly conduct
6 Which well that valiant courser did discern; 7 For he despised to tread in due degree,
in due degree > carefully, slowly
8 But chafed and foamed with courage fierce and stern,
stern > stern; terrible
9 And to be eased of that base burden still did yearn.
still > ever