When the Ghost Dog Howls by R. L. Stine
Andy and Marnie think they've won big when they get a free souvenir from a gift shop in HorrorLand. So what if this souvenir happens to be a rare (and possibly haunted) hound's tooth? It's totally worth the risk to have a gift that grants wishes. But when Andy hears howling at night and Marnie starts acting strangely, they realize that they might have to pay a price after all...