In the "Gator Ghoul" episode, Scooby-Dumb is introduced as Scooby-Doo's cousin, but in "The Headless Horseman of Halloween, Shaggy introduces Scooby-Dumb as Scooby-Doo's brother.
This is the second of 4 episodes that Scooby Dum appears in.
Shaggy: There he is, old pumpkin puss!
Velma: The hands came from the wall.
Shaggy: I've heard of walls having ears but hands too?
Shaggy: I'll remember this Halloween night for the rest of my life...IF I have a rest of my life.