A Pup Named Scooby-Doo - Season 1 - Eps 2: The Sludge Monster from the Earth's Core
The Coolsville bank is robbed. The janitor says that he saw a monster in the vault but nobody believes him and he is accused of the robbery. He's proven right when Scooby finds the monster inside his doghouse.
About A Pup Named Scooby-Doo

Title: A Pup Named Scooby-Doo
First Air Date: 1988-09-10
Last Air Date: 1991-08-17
Status: Ended
Rating: 7.6/10 (from 275 votes)
Language: EN
Seasons: 4
Total Episodes: 30
Network: ABC
Genres: Animation, Mystery, Comedy, Family, Kids
Production Companies: Hanna-Barbera Productions
Synopsis
The world's favorite chicken-hearted canine, as a puppy? That's right! And the old gang is back with him. Shaggy, Daphne, Velma, and Freddy are all here as gangly kids — goofing off, solving kid-size mysteries, and having run-ins with ghouls, ghosts, and goblins.
Cast

Don Messick
Scooby-Doo (voice)

Casey Kasem
Shaggy Rogers (voice)

Carl Steven
Fred Jones (voice)

Kellie Martin
Daphne Blake (voice)

Christina Lange
Velma Dinkley (voice)

Scott Menville
Red Herring (voice)