Boom is a baby Thunderdrum that Thornado is currently taking care of. Boom's name was originally going to be Lloyd as chosen by Tuffnut but was changed by Fishlegs to make more sense. But Tuffnut still wants his name to be Lloyd, or Darren. Like Bing and Bam, they were chased by wild dragons from Dragon Island to the sea stacks.
According to the Book of Dragons in Dragons: Rise of Berk, Boom is the youngest and most undisciplined of the triplets. And Snotlout found that out...the hard way.