Found some wonderful artwork by character designer Nicolas Marlet and various other artists that worked on the concept art of How To Train Your Dragon on another blog I follow: Here
The blog is called Living Lines Library and has lots of concept art, animation tests, storyboards and more from all sorts of animated movies.
There are also lots of interviews with various people that worked on the film on Dreamworks' YouTube Channel.
Also found a rumour (I say rumour because I can't find any evidence to support it) that a domesticated mountain lion owned by the one of the directors' (Dean BeBlois) uncle was the inspiration for they way Toothless moves! Found on wikipedia, under the Development section.
Production development blog for How To Train Your Dragon.