The MUTOs are ancient parasitic Titans that evolved and thrived in the same prehistoric ecosystem as Godzilla and the other Titans. The MUTOs reproduced by killing members of Godzilla's species and laying their eggs inside of their prey's radioactive bodies. At some point, the majority of the MUTOs died off, but not before a MUTO Prime injected her eggs into Dagon, one of the last known members of Godzilla's species. The eggs slowly gestated inside of Dagon, steadily draining him of his nuclear-rich blood, which eventually killed him. These two eggs would go on to hatch a male and female in the year 2014.
After the death of these two MUTOs, the same MUTO Prime that had laid them as eggs resurfaced and attempted to produce more MUTOs by injecting eggs into Godzilla, though it was defeated by the latter.
In the year 2019, a second female MUTO awoke and began to rampage while under the control of King Ghidorah. After Ghidorah's death at the hands of Godzilla, strangely, this female MUTO submitted and bowed to Godzilla, rather than attack him as the two MUTOs in 2014 did.