That is Gojira (which is actually how you pronounce "Godzilla" in Japanese). And it makes a whole lot of sense that a game about a nuclear post-apocalypse would have its own a giant radioactive lizard. The monster was a callback to earlier Fallout games, with the difference being that those games featured relatively crude graphics that could not render a gecko's goofy walk in such high-definition glory:
Gojira is a bit of a contradiction. On the one hand, he's a silly, bug-eyed gecko who is, for some reason, made out of rat meat. On the other hand, he's a fire-breathing maniac who'll never stop killing, and has such ridiculous stats that you couldn't take him down with all of the nuclear warheads in the game combined.
While the developers may have hid him out of shame, modders found him and immediately brought him up to the surface. Hopefully they were soon stomped to death and learned to regret their own hubris, thus completing the moral arc of every Godzilla movie.