I'm sorry that you dont have this at the moment. Storm8 is rolling this out to everyone gradually. You may want to check if you are using the latest game version on the Playstore, e.g. Android version 1.8.0.1g, and see if you have it. And if not, I hope you will get this very soon.
The Colosseum is like our tournaments and battlegrounds, you get a win the prize animal at the end of many series of successful battles, 335 items. You get streaks to help you to get more items, see Chapter Colosseum for more information.