From the Super Mario Wiki, the Mario encyclopedia
Lord Fredrik's horn in Donkey Kong Country: Tropical Freeze is an instrument with magic properties, most notably having the capability to change its size and powers along with its owner. Lord Fredrik equips the horn in its normal state by wearing it loosely on a string around his neck, as it can be seen during the final duel; however, he can blow into it to increase its size considerably, along with his own size. The horn is sturdy, made out of wood and is bound by two metal rings encrusted with glowing blue gemstones.
In the game's opening cutscene, Lord Fredrik is brought the horn by several Pointy Tucks after Donkey Kong Island is spotted on the horizon by the Snowmads. He blows into the horn to summon a windy ice dragon, which turns the island into an arctic wasteland and blasts away the Kongs, its main locals, to a foreign island.
The horn is seen again, much later, during the confrontation with Lord Fredrik in Volcano Dome. He uses it to attack from the background, mostly shooting small ice globes that contain enemies onto the battle arena. In some of his attacks, he also uses the blowhorn is used to send ice dragons, similar to the one in the opening cutscene.
After Lord Fredrik and the Snowmads are defeated, Donkey Kong takes hold of the horn, which shrinks down to fit his size, and blows into it, producing a breeze of flowers that warm up and restore the island to its former climate. Moments later, he uses the horn again as a saxophone when the Kongs form themselves in a band to celebrate their victory.
The horn displays similarities to Kaptain K. Rool's blunderbuss from Donkey Kong Country 2: Diddy's Kong Quest. Both of them harness abnormal qualities and are wielded by main villains. The horn also alludes to Mjölnir, the legendary hammer wielded by the Norse god of thunder Thor; amulets modelled after the hammer were widely worn as pendants during the Viking Age, much like Lord Fredrik dons the horn.