Billy

Billy is a character that was supposed to be added but ended up only appearing in the game as a Brick on wheels for "Easter Fools 2018" with the Audio file if you got close enough saying, "Thomas," puffed Billy "stop telling me what to do, you are a very bossy engine."

Version 1
Version 1 was built by FfarquharDeveloper, the Audio was Provided by TrainKidTyler, initially, Billy was a ThomasBlox3D release but that was canceled.

Version 2
Version 2 was the Actual Version of Billy, only Developers can get the model however the lack of detail was heavy and it was canceled. He can be found on one of the thumbnails featuring a lot of engines.

Version 3
Version 3 was supposed to be the actual Billy Model. Features include a 3D face, more detail, and detailed siderods. However this model was never finished and no screenshots were ever taken of it. Only Shadophy, FfarquharDeveloper, and undertale_andbatimAT have seen the model.

Charlie
Also, Charlie is a cheap reskin of Billy, meaning Ffarquhar refuses to add him into the game.