The Headless Horseman costume is the same as the Headless Horseman in the The Scooby-Doo Show episode, "The Headless Horseman of Halloween" from 1976
The dinosaur costume that Shaggy wears is the exact same one worn by Dino Man in 2016's Lego Jurassic World: The Indominous Escape
Cassandra Peterson, who voices Drella Diabolique, is the actress who portrays Elvira, Mistress of the Dark.
Grimsley Mansion made its original appearance during the first Lego Scooby-Doo short film Knight Time Terror (2015)