Every time you complete a task for Grizzly Adams and the creature has a boss you get the right to kill that boss once.
If you ask another task for the same creature without had killed the boss your quest log will be updated and you lose the right to kill it. But if your new task requires to kill another creature you keep the right to kill the first boss. Keep in mind you can hold 3 different tasks at the same time.
This dynamic changes after you reach the top rank of Paw and Fur Society's with 100 task points. Now, every time you deliver a task for a creature you earn 1 boss point.
There is no limit to accumulate boss points and you can redeem the right to kill any boss of you level range exchanging 1 boss point. The boss points will be deduced from you only if you already killed that boss before, because when you have the top rank, Grizzly Adams gives you the right to kill any new boss for free.