They resembled two men in skeleton costumes, because 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.