The chomper: The pieces that you move disappear mysteriously with the chomper in it's place on your move.
Invent the new worst/most useless chess piece ever!

The neutrin. It's barely visible, and can travel to any square (even if occupied) in one move even if the path is not straight, or is blocked by enemy or friendly pieces. However, it can't capture, deliver check, nor be captured. Similar to the ghost piece, but does require belief in the afterlife.

Many of the suggestions for new pieces are creative and funny, but not useless. For example, my own suggestion, the "neutrin" (shortened from neutrino) is actually very useful since it would allow a player to pass on their turn -- something not normally possible.
Now the "boy band" piece, that sounds useless

The King. If you can't prevent your opponent from being able to capture it on the next move you automatically lose the game, no matter how many other pieces you have.

the doctor, it moves around the board, and jumps into the past and drags up old pieces(that have been taken) form foe and you alike.

the pauper. it has no royalty, and sides with what ever side can put a 'royal' piece next to it. i.e. knight, king, or queen

The Rat is far from useless !
How about "The Moderator". Every move by the king or queen is regarded as political and every bishop moves is regarded as religious. Game is forfeit when on the board !

how about the invisible man, it cannot take anything, move anywhere and resides outside the board, invisible

The Blunderaptor. Rapid and efficient blunder killer. He's a traitor and serves your opponent though.

GrThe grey goo.
The grey good starts anywhere, determined by black. Starting with move one, each player plays a legal move as normal but ends his turn by creating a piece of grey goo next to any empty square adjacent to grey goo. If there are no empty squares next to any grey goo on the board then the grey goo can capture any piece including the opponent's king (or your own king). It is possible to capture the grey goo, but you still have to place down another piece at the end of your turn. However, kings cannot capture grey goo. If they entire board is filled with grey goo and kings then the game ends in a draw. Any player who only has a king who is surrounded by grey goo ends the game in stalemate.

He dances
He eats ur peaces
He suicides
Ur elo gets to"0"
He sings
And best of all..............THIS IS SPARTA!!!!!!

Here's the most useless piece for you. The "Fiveshop". It replaces the Bishop. You get 2, the light-squared Fiveshop and the dark-squared Fiveshop. It must move exactly 5 squares diagonally and can't jump.
Therefore, the one that starts on c1, it has 1 legal move, Fh6, and can only be played when d2, e3, f4, and g5 are open, and h6 is either open or has a Black pawn or piece.
Once it has moved to h6, it again has 1 legal move. Fc1.
Because it must move 5 squares diagonally, all it can legally do is go Fh6-c1-h6-c1-h6-c1 etc.
Same thing goes for the other 3 of them. Fc8-h3-c8-h3, Ff8-a3-f8-a3, and Ff1-a6-f1-a6.
There's your "most useless chess piece ever!
Addendum: If you want to make matters worse, make it is "Sixshop", it must move 6 squares diagonally, which means it never has a legal move, and all it does is block the development of your rooks, possibly protect your king along the back rank, and makes castling physically impossible unless your opponent captures the Sixshop and then moves back out of the way, and you still haven't moved your King or Rook!

The block. It is located in front of the rook pawns. Can't be moved. It makes the king immune to checkmate unless the king is under check XD.

The Corpse. After one of your units is captured, it is replaced with this flat disk on which any unit, yours or your opponent's, can stand or move across. There exists a special move like castling called Cremation by which either side may remove The Corpse from the board, but it costs a tempo. Why would either side make that Cremation move? Because if you don't, any unit that has stood on that square after 5 moves develops typhoid fever and must be automatically and instantly removed from play.
The extra ghost. Outside of the board, it is used if you promote to a ghost...except you can't, since ghosts can't move!