A lab experiment

Unknown to normal society scientists in a mysterious facility decided to mix the DNA of every inhuman humanoid they could get their hands on and put it all into the egg of a human. They studied it as it grew in the incubation tube they had it in. Over time they realized that their experiment wasn't quite stable they would start anew. They kept the one they lived of the original batch alive to experiment on to see what all they needed to fix to make it stable.

In the twenty-two years of life that their experiment had it had never woken, or even come close enough to waking to have conscious memories. It was on the night of a really bad rain storm someone they couldn't catch broke in to release the experiment from it's long unnatural slumber. The security chased the intruder through the entire building, but never came close to catching them. Because they were focused on catching the intruder no one noticed that the machines keeping the experiment asleep had shut off. Or that she'd broken free and had ran off into the night. Least not until long after the storm had stopped and they had gotten the power back on.