The Giant Rat manifested at a New York Zoo and terrorized elephants in their exhibit.
In a zoo elephant pen, a Giant Rat scared all the elephants and cornered them. The Ghostbusters arrived on scene but Peter, as Venk-Man, went in alone, overpowered the rat, and wrestled it into submission. This was considered Venk-Man's public debut and he was ushered to appear on various talk shows. Presumably, the Giant Rat was handed over to the Ghostbusters to trap and place in the Containment Unit.