The Fireworm dragon is a Stoker class dragon that appears in the Riders of Berk episodes Race to Fireworm Island and Portrait of Hiccup a Buff Man.
They are small dragons who often hide in dark caves. They seem to be small with little wings (although it was previously believed they have no wings due to the concept arts showing them wingless and previous episodes didn't show enough of them). These dragons have skin that burn hotter than the sun.(However, if this is true, anything they touch will disintegrate because the sun is 5,778 Kelvin, or 5,504.85 Celsius.) All fireworm all are born and reignited on fireworm island
In the Defenders of Berk preview, the large golden Fireworm appears to the Queen and it has a much larger body-to-wing ratio.
As accidentally discovered by Ruff and Tuff, the closer they get, the brighter they burn.