Thornado (a combination of Thor and Tornado) is Stoick's Thunder Drum who first appeared in the Dragons: Riders of Berk episode How to Pick Your Dragon on October 3, 2012. He was named for having the "power of Thor" and the "ferocity of a Tornado", "besides it's the only thing he responds to."
In Riders of Berk Thornado is first mentioned as a rogue dragon wreaking havoc on the Hairy Hooligan's fishing ships. Stoick and Hiccup, on Toothless' back, find it while it's battling Mulch and Bucket and blowing their ship to pieces with sonic blasts. Stoick challenges the dragon, and drives it into the water. However, Thornado's tail grabs Stoick, pulling it down with him. Stoick is able to grab the dragon, and Hiccup, Toothless, Mulch, and Bucket watches them wrestle.
Admiring the Thunder Drum's powerful fighting spirit and spunk, Stoick traps Thornado in a net and declares he wants him trained as his own dragon. The next day, Thornado's mouth is muzzled, and Hiccup instructs Stoick to make sure the dragon can trust him. However, Thornado won't let Stoick touch him, and Stoick gets frustrated and wrestles the dragon once again. Stoick tries to ride him, and they tumble away from Berk. Thornado manages to throw Stoick off his back and escapes to an island. Hiccup and Stoick gathers the gang, and they try to find Thornado.
Hiccup and Stoick finds Thornado, and they find out he had been protecting and feeding his friend (why he was stealing their fish), a violet thunder drum with an injured wing hidden in a small cave. Stoick tells Hiccup to get the gang to help him, while he confronts the dragon. After Hiccup and Toothless leave, Stoick tries to reassure Thornado, but a group of hungry wild boars arrive to prey on the injured dragon. Stoick, later with help from Thornado, fights the boars. After they take out the first wave of boars together, Stoick and Thornado come together to protect the injured Thunder Drum and bond at last. The second wave of boars charge, but Stoick removes the muzzle, allowing Thornado to blast the boars with sonic power. Hiccup and help arrive, and they rescue the injured dragon, bring it home, while Gobber treats its injuries. Thornado then goes on to assist Stoick in his chiefing duties.
In Portrait of Hiccup as a Buff Young Man Stoick rides Thornado while searching for Hiccup. Thornado's only appearances are waiting for Stoick and Gobber on the beach, and standing by the other dragons while the kids try to save Hiccup.
In Dragon Flower Thornado appears more in this episode as Stoick rides him to the Shivering Shores while revealing his name. It is also shown that while Stoick has earned his trust, Thornado is very stubborn as he tries to buck Stoick off him, much like Snotlout and Hookfang, but Stoick has the strength and tenacity to stay on and command Thornado. When Stoick and Thornado return, the Thunderdrum goes ill by the Blue Oleanders that Mildew planted. Stoick immediately rushes to Thornado's aid by comforting him, putting a pillow under his head, and gets an anti-venom from the Scauldron. Thornado is cured after the ordeal blows over.
Thornado has made brief appearences in What Flies Beneath, Defiant One, Breakneck Bog, Gem of a Different Color, and We Are Family Part I. In We Are Family Part II, he and Academy dragons work together to save Hiccup and Toothless from the Outcasts.