The theory goes like this:

Toothless, and by extension night furies, is/are alpha dragons, like the bbeg's in the first and second movie. Toothless was being raised by the queen to take her (it's?) place.
EVIDENCE
1. Aquatically built
looking at the queen from the first movie, it was clearly meant for water. Dark blue, coral-like frills, etc. The bewilderbeast just lived underwater. Now, if I'm not mistaken, Toothless, if I'm not mistaken, is more amphibious looking than reptilian. A mature axolotl, anyone? A newt?
2. Moarchs
Toothless can control other dragons, as movie 2 demonstrated. so could the other alphas
3. Other stuff! Yay!
-powerful breath weapons
-every dragon is terrified of Toothless
-Toothless is seems to be smarter than your average dragon
-the point of the movie(s) was to illustrate the similarities between Hiccup and Toothless. Much as Hiccup was being raised for Cheifhood, so was Toothless

STRAWMEN (YAY?)
What about size? Toothless is pretty small.
so is hiccup. as I said, Similarities.


That's all for now.