Chupon only appears if you bet a weak item, like a potion. You have to bet better items and equipment to fight anything other than Chupon. If you talk to people around the colisuem, you'll see that they say that Chupon appears to get rid of people with dinky wagers that are wasting the coliseum's time.

For a list of what you can bet and what can be won, check out: http://www.eyesonff.com/ff6/coliseum.shtml