So you want to make a truly unique challenger in the arena. To figure out what you can play lets start with the basics. The basic CR rating for the battles is CR10. a 11th level creature without HD is a CD10 creature (class levels equal CR -1). So any CR10 creature can be used without modification.
You can also use monster templates on lower CR creatures to bring them up to CR10.
Another option is to use a creature with HD then add class levels to bring them to CR10.
Finally if you feel really adventurous you can use a combination of HD creature, template and class levels to get to CR10.
Below is a list of creatures and attribute bonuses they get for level advancement to CR10.