Anguirus

Anguirus (アンギラス Angirasu) is a giant Angilasaurus kaiju who first appeared in 1955. He was originally an adversary of Godzilla, before being beat by Godzilla and later becoming his companion throughout his journeys.

Origins
It is suggested that Anguirus is an Angilasaurus that lived during the Cretaceous, 135 to 70 million years ago, around the same time as Godzilla. Like Godzilla, Anguirus managed to survive into the modern day by lying dormant before being awakened and irradiated by nuclear bomb tests in the 1950's. The First Generation Anguirus specifically is said to originate from Siberia, according to the GDF.

History
Anguirus was awaken from his slumber in Sibera and proceeded to head towards Iwato Island, due to sensing Godzilla's presence there.