Shinomura (which translates roughly from Japanese to "Swarm of Death") is a Kaiju sighted along with Muto and Vishnu, and is one of the first Kaiju ever to arise after being attracted to the surface by the Hiroshima bombing. He attack ships and nuclear repositories across the world... until it was eventually taken down. Shinomura appears to be similar to Hedorah and Destoroyah, as it is, in fact, not one animal, but comprised of millions of tiny creatures that work together as an aggregate to form a giant Kaiju, and all they need to reform the colony is radioactivity.
Abilities/Aspects -Flight speeds up to Mach 7, creating sonic booms -Jagged jaws and mandibles -Barbed tail -Capable of splitting into smaller forms
Ancient past was full of radiation eating creatures. Due to high energy consumption and bizarre biology, some of them could grow gigantic to kaiju proportions. Thus the kaiju are all from the distant past. Shinomura, the MUTOs, and Godzilla were three species of these ancient kaiju. However the Permian-Triassic extinction event wiped out 90% of all life on Earth, forcing the surviving kaiju into hibernation, often near volcanoes or at the bottom of the ocean to siphon off radiation from the planet's core.
In 1945, the Hiroshima blast reawakened a fragment of Shinomura, which rebuilt itself over some years until 1954, when the Nautilus submarine woke up Godzilla. Godzilla and Shinomura fought until they were both lured to the Bikini Island test range and nuked. Shinomura assumed dead but Godzilla re-emerged several years later, but was reclusive.
1999 the mine in the Philippians breaches a cavern that reawakens a Male MUTO, and then the movie proceeds from there.