From Battle of the Immortals Wiki
Most monsters in the game can be captured and used as pets. Capturable monsters will have a small ico] to the right of their health bar:
When a capturable monster is defeated, there is a small chance that it will immediately respawn as a non-hostile pet-type monster. It will have an always-visible green name (like a pet), and will not move. To capture this monster, simply left-click on it and wait for the timer (about 5 seconds). This timer will be interrupted if the player takes damage or moves (like when summoning a pet or mount), so it may be necessary to clear away nearby hostiles before capturing.