There are actually two bonuses you can get for Monster Hunter Stories. The first requires your 3DS to have save data from the Monster Hunter Stories Demo, and the second requires save data from Monster Hunter Generations.
To get the Monster Hunter Stories Demo, all you have to do is start up the eShop and download it for free from there. If you decide to get the full game, all of the saved data from the Demo will transfer over. Just make sure when you start the full game you choose to use the Demo saved data if you want to keep it.
Navirou is an odd Palico that accompanies you in Monster Hunter Stories who can wear different outfits. By playing the Monster Hunter Stories Demo first, Navirou will get a Rider’s Outfit to match you.
If you have Monster Hunter Generations save data on your 3DS when you start playing, Navirou will gain another outfit. This is Hellblade armor, based on the armor that can be crafted from the Hellblade Glavenus in Monster Hunter Generations.
Additionally, you’ll get a Rustshard, a material you’ll be able to turn into a powerful Glavenus Sword & Shield a bit later in the game.