They resembled two men in skeleton costumes; it was obvious that they weren't real because of the gray areas between the bones.
They walked around waving their arms and emitted ghostly laughter.
The New Scooby-Doo Movies
The skeleton disguises were Joker and Penguin's final attempt to scare the gang out of the haunted house at Gotham City Amusement Park, which they were using as a hideout, when the rest of their tricks and traps failed to do so. The disguises ended up backfiring; Scooby-Doo mistook them for "delicious-looking bones" and started chasing them. In their efforts to escape him, they fell into their own trap, which they had previously used to capture Batman & Robin. The dynamic duo unmasked the villains, and they were arrested shortly afterward.