ComPet

The battle area is an upcoming feature for Entropia Universe on top of the "Taming" system.

Development history

The battle arena was first mentioned in the development roadmap in May 2014:

"Web-Based Battle Arena - (Planned for Early 2015 Release)

A web-based Battle Arena game is planned for early 2015. The game will be developed as a cross-platform solution available on the web and for mobile devices, and will have direct ties to the Entropia Universe taming system. Players will have the opportunity to acquire, upgrade and customise their pets for battle against the pets of others.

Entropia Universe participants will have the ability to acquire Battle Arena Deeds and share in the revenue and excitement of the Battle Arena action. 200,000 Battle Arena Deeds will be offered via the Entropia Universe in-game auction and upcoming webshop at 100 PED each. Revenues generated from the Battle Arena Deed offering will be used exclusively for Battle Arena development and marketing. More detailed information about the Battle Arena Deeds will be made available very soon."

Source & discussion: http://www.entropiaplanets.com/threads/communication-and-development-roadmap.11555/