The Venomous Vorpent is a bright yellow Nanodragon that has a fatal sting.The cure to this sting is a potato, but they aren't on Berk in hiccups time. In How to Cheat a Dragon's Curse, Hiccup must steal the Potato from the Hysterics Tribe to save Fishlegs.

Actually, Hiccup is the stung one, Fishlegs just has a cold.