The Costume Party Ghosts were a large group of identical ghosts (whose species prior to their deaths are unknown).
These two ghost friends, usually appearing as wispy bedsheet ghosts with only the most basic of faces, wearing a cap and a hat respectively, were among the monstrous and supernatural guests staying at the Hotel Transylvania in October 2004. Their nature had given them a warped idea of what was strange and scary, and they ordered "headmaster" and "dentist" disguises in anticipation of a Halloween costume party, intent on scaring folks with them. At any rate, Donald Duck provided them with the costumes, and they were quite satisfied.
Another, similar ghost, wearing no hat but otherwise very much resembling the two, was also staying at the Hotel, and was among the crowd of monsters, along with the other two, that insisted that the manager not fire Donald.
Behind the scenes
The Ghosts were only ever seen in the 2004 story Hotel Transylvania.