U mean a (cough!) Marlborough (cough!) don't u? The arena guy gives u a hint theyre to be found in the north of the Calm Lands. But they are rare anyway. I think somewhere in the North west corner is best. They are also found in the Sunken Cave but again rarely.

Another thing u can try is if after sometime u don't encounter the monster, go back save and then try again. The game gives u a slightly different set of random monsters after each save. Good luck!