Bolas are a type of weapon used to incapacitate dragons.
History
The Hairy Hooligans used them in dragon raids back when they were enemies. They used them on dragons again in the Riders of Berk episode, "When Lightning Strikes" when Mildew and a few other Vikings were hunting Toothless. When they believed that he was responsible for Thor's anger and making him strike lightning on Berk.
Bolas are used by Outcasts. In "We Are Family, Part 1", they used them to capture Toothless before another Outcast grabbed Hiccup. They were later used in "We Are Family, Part 2", when Alvin got Hiccup and Toothless to fly at him, while his men threw bolas to capture them.
Their also used by the Berserkers. In "The Night and the Fury", angered, after learning that Hiccup and his father lied to him, Dagur the Deranged used one to entangle Toothless in order to kill him but Hiccup frees him and defeats Dagur.
