This is book number 5 in the Bunnicula and Friends (Prebound) series.
It's Christmas Eve, and Howie the dachshund puppy is scared. Who is this big guy in the red suit who's going to come down the Monroes' chimney with a huge sack over his shoulder? Chester isn't scared of Santa; it's the Christmas "ghosts" he's worried about (and, of course, Bunnicula). So when there's a loud CRASH from the basement, he and Harold have to investigate -- to make sure the Monroes have a merry Christmas, not a scary Christmas.