From the Super Mario Wiki
Lord Fredrik's horn in Donkey Kong Country: Tropical Freeze is an instrument with magic properties that has 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 a dispatch of 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 Snowmads then settle on the island and conquer it.
The horn is seen again, much later, during the confrontation with Lord Fredrik in Volcano Dome. It is a ranged weapon used by the Snowmad king to attack from the background, shooting small ice globes that contain enemies onto the battle arena. Sometimes, the blowhorn is used to send ice dragons, similar to the one in the opening cutscene, to attack the Kongs.
After he is defeated, Lord Fredrik drops the horn and is cast out into the sea, crashing into the Snowmad fleet and destroying it. This results in the Snowmads evacuating the island. As Donkey Kong is overlooking the scene along with his partners, he notices the horn falling from the sky and catches it. Shrunken down to fit Donkey Kong's size, the horn is blown into and subsequently produces a breeze of flowers that warm up and restore the island to its former climate. To celebrate the victory moments after, the Kongs gather themselves in a band, with Donkey Kong playing the horn as a saxophone, unrolling the credits.
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.