The poison in the cyan phoenix was quite rare, so she wanted to take her back for research. With luck, she might be able to cure her. With this cyan phoenix's physique, if cured, she would be perfect for testing poison.
After all, she was really different since she could hold on for so long after being poisoned. However, if the cyan phoenix died, no one could blame her. She would just tell people that she was so severely injured that even the physicians of the Nangong Family couldn't cure her.
As for why she didn't use human to test poisons...
That was because human beings were not as strong as spirit beasts. Her poisons were so strong that a human would fall down dead as soon as they took any of them!
