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Loki is the Norse god of thievery and mischief. Although rarely mentioned in the books, Hiccup prayed to Loki when trying to steal a baby dragon. Also in between Dragons: Defenders of Berk and Dragons: Race to the Edge, Season 1, the twins dedicated their lives to Loki by pranking everyone on Berk.
It is shown that there is a holiday called Loki Day, which celebrates Loki's birthday by pulling pranks on others. When the prank is accomplished, the instigators shout "Loki'd!" at those who fell for their pranks.