The Pirate Ghosts are a trio of ghosts dressed like swashbucklers.
The Pirate Ghosts manifested at the Maritime Museum and stole a ship. The Ghostbusters were called and faced them. The ghosts chopped up the dock and forced the Ghostbusters to jump onto the ship. It soon set sail and the firefight began. The Ghostbusters wrangled them and Slimer captured them in three Ghost Traps each. However, the guys were stuck on the ship without a mast and ruder - just like their TV predicted thanks to a growing Water Elemental.
The Pirate Ghosts are Class 4 entities.