Oh, good, I thought I was the only one who noticed it! It's probably from a fight with another dragon, seeing as it looked to old for it to be from Toothless' fall. I bet they're going to tell us more about it in the sequel (or TV series. They're planning on one). It's probably important since they took the time in animate it in whenever his neck is shown.
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I NOTICED that too... He obviously had some kind of surgery...
Scar.
People think that the plaster on Marsellus' neck relates to the removal of his soul, which is wrong. In the close up shot of the back of Marsellus' head, Ving Rhames has a scar on the back of his neck that Quentin Tarantino thought drew focus, so he covered is with a sticking plaster.
The Scar was created in 2002-06.
Scar is the villain of the Lion King. Scar is the evil lion who killed Mufasa. Scar wants to be king.