The Old One (real name: Yibb-Tstll) is an ancient Lovecraftian deity and an enemy in the Real Ghostbusters cartoon series.
This hideous yellow and red tentacled fish-like beast with indigo eyes and a huge gaping mouth full of razor sharp teeth is a member of the Old Ones, a race of unnamed demonic beings from another dimension on the other side of the universe outside Earth's boundaries. They were worshipped by the Old One Cult, a secret society of fanatical humans who wishes for their return for an untold number of years since the Old Ones, or at least one of them, visited the Earth before in the distant past.
In order to keep the visiting Ghostbusters from interfering, they stole the book from a Soviet institute, and frammed the Ghostbusters. Meanwhile, the two met up with their fellow cult members miles outside of Dnepropetrovsk to summon the Old One from a hidden temple.
Fortunately, the Ghostbusters made it to the Old One Cult's subterranean temple compound and were able to stop it before it completely made it to the surface.
Slimer was sent back home to get "Big Trouble", a huge apparatus that uses the power of all the Proton Packs to create a rip in teh fabric pf space and time, like a black hole, to suck everything it touches. Peter Venkman bravely threw Big Trouble into the pit and ran way as he saw the Old One unravel its unpleasant visage. Then Big Trouble activated and it sends the Old One back to where it belonged in the end, while the Old One Cult themselves were immediately arrested at their temple lair by the police upon both their failed attempt to escape and their shattered dream of the Old Ones' return.