Godzilla Idea

If Gojira was real, a monster mutated by radiation and doing as it does, how could it be or work? Realistically? Or as real as possible?

Long explanation cut short...giant mutant penguin. Way more complicated than 'just' a big penguin... but atomic acidic vomit? Upright walk? Covered in quill-like scales? Has the genes for claws and teeth? Territorial? Aquatic? Smart? Mean brows? Can grow clear spines that could glow or reflect blue light?
BIRD.
Aquatic flightless sea bird. Polar bird who can thrive in cold waters.