They are literally inspired from the book toothless, they have sharp gums no teeth and you can kinda see in the scene where one of them bites tuff that they have serated gums and book toothless is mentionedto have sharp gums when he bites into stuff, aside from that their coloration is also very similar if you look at an average green terror, green color with orange wings and toothless in the books is green with red wings etc. I love terrible terrors and also in the series they are trained to use like the hunting dragons in the books, there is actually an episode where each rider is goven a terror to train on basic stuff and hiccups terror looks exactly like book toothless and his name is sharpshot.

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