The Neo-Soviet marine biomods
Classified as a category 4 threat, the Elib is somewhere between a vampire lord and Godzilla for limits of escalation. Yes, I would consider using a nuclear device to kill just one. I am that sure that I want it dead.
A circus is in town and while parading down the main street the animals suddenly break free from the cages. After surviving and assisting during the breakout the PCs are hired to help with the investigation. Who did this and why?