Oh good, sineala is on the case already. She was the first person I thought of as likely to have ideas.

I have thought of another way to do it: look up the (putative) etymology of Howard and find roots that you can mangle use as a basis to come up with a new name. Helpfully for you, one website gives three possible etymologies: http://www.behindthename.com/name/howard - 'ewe-herder' isn't immediately pregnant with possibility (although you can probably get in a great insult about 'fit only to...') but the links to the Germanic and Norse versions are more hopeful for a gladiator - Latin nicknames for 'brave' (one of the Germanic elements) or 'guardian, defender' (Norse) must exist.