Premise: Casper's First Christmas is a 1979 Christmas television special and crossover produced by Hanna-Barbera featuring Casper the Friendly Ghost and his friend Hairy Scary from the animated series "Casper and the Angels" with guest stars Yogi Bear, Boo Boo, Huckleberry Hound, Snagglepuss, Quick Draw McGraw, and Augie Doggie and Doggie Daddy.
Hypothesis: Hanna-Barbera didn't include Scooby Doo as a guest star for Casper's First Christmas.