This page offers a guide on how to complete A Beast’s Burden Mystery and obtain Gullinbursti the Boar as a mount in the Dawn of Ragnarök DLC.
Where to Find Gullinbursti (Mount)
Gullinbursti the boar mount and the A Beast’s Burden mystery are found on the northern side of the bridge in Svaladal, north of Fitri. As you approach, you’ll be greeted by a Jotun Warrior riding on Gullinbursti. He is immediately hostile to you and if you want to free Gullinbursti, you’ll have to defeat him. He’s tanky but not that strong - he will, however, use the boar to his advantage. He can charge at you and use his long reaching javelin to poke you down.
Don’t worry about where you hit, hitting the boar will actually damage the Jotun warrior. We tried using the harpoon to pull the Jotun warrior off the boar but it didn’t work so it looks like the only way to get the mount is to kill the warrior first.
Once he’s dead though, Gullinbursti will automatically become your primary mount. He’s reasonably fast but doesn’t feel as agile or smooth as a horse. He does look very cool though!