Despite how calm the king of water beasts looked, the truth is that she was furious, because she is smart enough to know that the four major races coinciding in the same place like this is essentially impossible to happen jut by luck.
Also, now that she looked at the black horse that appeared at the front of her main force, which is the sole reason as to why she decided to be on the move and not just send a part of her forces to go after him, she realized that it wasn't that strong.
The explanation behind the previous is actually quite simple, the liquid in the container which the little wyrm poured on the black horse along the way to the living area of the reticulated boa race, was a mix of Vivian's blood and the venom of the black horse.
