The hesitation Toothless shows during the famous "touch" scene was actually an animation error. However, it looked so perfect that the animators chose to leave it in the film.
The Night Fury dragon species was originally going to be more wolf-like in appearance. Instead, a DreamWorks employee's computer screen saver of a black leopard inspired the film's creators to make Toothless more feline in appearance.
There is a drawing very closely resembling the Flux Capacitor from the Back to the Future (1985) movies on Hiccup's design board.
In the book of "How to Train Your Dragon," Hiccup does not have a love interest. Furthermore, Toothless, the dragon in the book, is about the size of the Terrible Terror breed, and he is also green and red, not black. Toothless also got his name because when Hiccup first found him, he had no teeth. He grew one tooth, only to lose it shortly later. The producers decided, with the approval of author Cressida Cowell, that it would be more cinematic to make Toothless large enough to be ridden as a flying mount. As such, Toothless was completely redesigned as a rare Night Fury, a highly intelligent breed of dragon evolved for speed and stealth. His personal name, in the film, derives from Hiccup's observation about how his teeth are normally retracted into his jaws so they don't interfere with his fire breathing, which is typically projected as an explosive pulse.
The appearance and personality of Toothless was inspired by cats, dogs, and horses. The face also bears some resemblance to the giant salamander, the largest amphibian in the world. The shape of his face is also reported to have been based on the face of Stitch from Lilo & Stitch (2002), also directed by Chris Sanders and Dean DeBlois. Sanders and DeBlois used to work for Walt Disney Pictures as studio members until 2007 when they joined DreamWorks Animation to work on most films starting with Kung Fu Panda (2008).
David Tennant: the narrator of the How To Train Your Dragon audio book series, provides the voice of Spitelout, though he is not credited. He also narrates some features on the film's website.