Goblin Monday (Goosebumps House of Shivers #2) by R. L. Stine

Mario is spending winter break with his neighbors, Todd and Jewel, on their family vacation to Vermont. He is excited to see snow for the first time and meet his friend’s beloved grandparents. But Todd and Jewel’s grandparents turn out to be a little odd. They love telling tall tales of goblins, elves, and trolls. When Mario spots a blur of green fur in the garden, he wonders if the stories could be real. He’s determined to find out more, but the truth might be more dangerous than fiction.