The Piece Inventor.
When it's checkmate, the Piece Inventor invents a piece that can save the King.
You can capture a Piece Inventor, but it can afterwards invent another rule why in that position the Piece Inventor couldn't be captured. And the position will be reverted and you will lose for doing an illegal move.
It's got to be the Staff. Can only be used by a pawn, however, it never moves itself, AND is setup on either the first or eighth rank....
The Piece Inventor Inventor. When captured, creates two Piece Inventors that can be placed anywhere in the 3rd to 6th ranks.
Piece Disinventor: Can delete any chess rule except about king once per move. If you capture it, can afterwards delete rules for that capture, revert position and you will lose for doing illegal move.
Bodyguard: stays on d1 until enemy's king is checkmated. After that, he can capture all opponent's pieces in one move, excluding king!
Best one until now! Keep it up 
The minichess: a small separate chessboard piece: first action is done on small board (starting position is as on standard chessboard and then same action with small chessboard piece on large board (on castling 2 moves right), can moved by any side, but only owner can do capture. If captured, game is played until win, after that, piece is lost and chessboard after win transforms into winning side's rook, but if drawn, piece normally captures. Transforms to winning side's rook after any side won, but if drawn, chessboard disappears. Also on small board you don't need to go with your king out of check.
The Pawn King: It spawns 9 pawns a round it every turn and moves all pawns when it moves
The McGregor piece. If you try to move it it swears and gives you the finger.
Computer: does nothing but allows Hacker hack that game!
ujhguygyuugyuygugyguy: it forces you to resign in five moves, if you don’t resign it will turn into a bomb and explode in no time and kill you
The random piece that does nothing
The king's pet. It occupies 1 square and moves like the king. Follows the king behind. Can't capture, can only block and die
the reseter- every time you move your queen or your pawns, it beeps 100 times then resets the board.
pikotaro
annoys you to death