They've been irregular for a long time, probably at least a thousand years, it is hard to say how accurate their histories are.
Real-life Direwolves weren't especially large.
Shadowcats = Some variant of tigers, jaguars or leopards
Lizard-lions = Alligators
Krakens = Giant Squid
Targaryens aren't the only ones to have Valyrian features like violet eyes and silver-blond hair. It is just that most of their people died when their island nation when kaboom and took out most of the surrounding area.
Not all Lannisters are blond. Their genes are actually suggested to be recessive, but George has also admitted that genetics is not a strong suit of his and the books are not scientifically accurate.
Radiation kills stuff. As much as comic books make it sound great, DNA rarely benefits from irradiation.