Egon's Dragon

After finishing their latest job, the Ghostbusters accidentally wake a large dragon from its sleep at the bottom of a dried up well. The dragon was accidentally created/summoned up hundreds of years earlier by Zedekiah Spengler (Egon's ancestor) and mistakes Egon for him. What's more, when the dragon was first put to sleep, it was the size of a small horse. After centuries of slumber it is now the size of an office building.