Once you rescued Meta Crab from the kids at the Harbor, after reaching Step 6 at Snow Mountain, you can get your Surprise Horn back by scaring the kids.
You need to be level 20 and know Vanish. Build and equip Vanish and talk to the blue haired kids in front of your house in Rococo twice.
If you talk to one of the Police Officers beforehand, he'll tell you that the kids are terrorising the city.