The zombie:
A piece that can be an extra queen that can be castled (in place of a rook) but if you do a carless mistake, you become a zombie. (the piece that was carelessly lost becomes a zombie piece)
The Bee
Can replace any piece on a regular game (except for a king), gains the replaced pieces moves and captures, but, if a piece goes behind it (from the person who has it's perspecitve) than both the bee and the piece dies.
The square adder: adds a square to the chessboard randomly.