The Tamed Betta Fish is a special critter that you could win in a Daily Event.
The purpose of the event was clicking on a poison machine in Tanglevine Jungle where the river is. If you complete the game of fishing you will get some things that looks like bubbles. They will show up in you Loot, there will be some black poison in the water stand the poison and click the bubbles in your Loot. Once you clear all the poison away it will tell you what you had done and give you a Tamed Betta Fish.