The Kaiju are a race of amphibious creatures genetically engineered by the Precursors, a sentient race from the Anteverse. In 2013, the Precursors opened a portal between dimensions at the bottom of the Pacific Ocean, allowing the Kaiju to enter Earth's dimension. (Source:pacificrim.wikia.com)
Então é claro que, Kaiju é o monstro criado por cientistas alienígenas.