In the world gooroo call home, there are monsters and there are beings, and where beings are often weaker, they are ingenious creatures capable of finding ways to defeat monsters far more powerful than themselves. However, there is a point when the tools of war make beings into monsters.
Gooroos are a form of advanced, sentient dungeon slime that has learned how to live among, and sometimes prey upon people. Mochi's realm name has been lost to time, but she was a passive feeding class gooroo which meant she survived on the pyschokinetic energy of dying beings without actually seeking their deaths. Unfortunately, she was in Hiroshima the day the bomb fell.
The massive number of deaths during and after the drop completely saturated Mochi with energy, and this combines with the the radiation and the fallout cause her to grow rapidly to an incredible size before it actually induced a fission event that produced the two known Mochi today.
The end product would inspire the idea of Godzilla on their world: virtually invulnerable, massive, and highly radioactive beings that almost nothing could stop/
For more details about the gooroos and Mochi, please see: https://fennecfox.net/sd_wiki/index.php/Mochi