Pocket Chess = Every player may take an extra unit, not necessarily
his own and not necessarily orthodox, and put this aside the board.
Both players can have different pocket piece, e.g. white could have a
pocket knight and black a pocket grasshopper. Which pieces each player
has got is part of the stipulation.
Once during the game, a player may put this extra unit on any free square on the board, instead of making a normal move. After this, this unit moves as is usual for this unit. All other rules are as in the orthodox chess game.
I also provide pointers to previous problems on this page