Drool is Curly the Skeleton's pet Bulldog. He only shows up on book covers and artwork where Curly is present.
He first appeared on books covers in 1995 on the cover of Monster Edition #1 and in the Tales to Give You Goosebumps series in 1996 on Even More Tales to Give You Goosebumps. His name was revealed in a summer 1996 Goosebumps Fan Club newsletter after suggestions were solicited in a prior newsletter. He is not an especially well-known character in the Goosebumps franchise and only shows up in merchandise if included in artwork that otherwise focuses on the much more iconic Curly.
A skeleton walking a skeletal dog makes a brief cameo in the film Goosebumps 2: Haunted Halloween, possibly a reference to Curly and Drool.
Drool appears during Goosebumps HorrorTown's Trick or Trap! event. He is asked to take over Curly's role as narrator when he decides to join the neighbors. When Curly feels down, he asks the other pets in town for advice, eventually receiving the perfect advice from Cuddles the Hamster.