User blog comment:Logo8th/Shadow Wing Dilemma/@comment-6800048-20170528005548

I'm not sure what the creators were thinking, mankind the two dragons look quite reasonably different from each other, as it messes up clarity. Unless the babies go through some significant appearance change in their transition to adulthood, I believe the species, while both can be called a Shadow Wing, are subspecies of sort that share a symbiotic relationship. I don't know how complex the ratio of babies born or whatever is and how the social structure works as the babies grow, but one can assume the specific individual Hiccup encountered just happened to utilise that form of attack.

Alternatively, you know, we could always go with the Titan theory but they're meant to be extremely rare so...