One of the chess variants, I used to play when I was a child was pocket knight, or paard in de zak, as this game is called in Dutch. It was, and still is very popular among especially young, but also older Dutch chess players. Pritchard mentions this game, and tells that it comes from unknown origins, from the end of the 19th century or the beginning of the 20th century. The game has been and is been played worldwide; Pritchard mentions tournaments in Great Britain, and mentions that the game has been played by prisoners in Russia during the time of Stalin.
Players take an extra knight, and put this aside the board. Once during the game, a player may put this extra knight on any free square on the board, instead of making a normal move. After this, the knight moves as a usual knight.
All other rules are as in the orthodox chess game.
Note that there are no restrictions to the square where the knight is placed, as long as this square is empty. Hence, it is allowed to put the knight on such a square, that check is given, mate is given, or, when a player is in check, between the checking piece and the king to remove the check.
The game is also played with only two knights per player. In that case, each player removes one of the knights from the opening setup (but both on the same line, i.e., either the knights on b1 and b8, or the knights on g1 and g8), and takes that knight as the knight `in its pocket'.