There is only one Kaiju, it was confirmed in an interview.
This chapter has a flash back where the guy is a child. Daikokuzan was a baby too in that time.
Knowing how ONE tends to work, Daikokuzan is still in its teenager years and hasn't reached adult form yet. That will be its "OH SHIT" moment when it is revealed this huge bulking current form is just a walking cocoon and his real end-game form is sleeping inside. Sometimes, it's moving a bit in its sleep and that causes a nuclear explosion.