Shō shogi (小将棋, "small chess") is the 16th-century game modern Shogi grew out of. It is played on the same 9×9 board, with the same array, plus one piece: a Drunk Elephant (酔象) in front of the King, which promotes to a Crown Prince (太子, kōtaishi) - a second King.
Two things changed on the way to modern Shogi, and they seem to have changed together, around the reign of Emperor Go-Nara (1526-1557): the Drunk Elephant was dropped from the array, and captured pieces began to be returned to play. This game offers the two states of that transition, and asks which one you want before the first move:
Everything else is the same in both: the same board, the same pieces, the same promotions.
The Drunk Elephant moves one square in any direction except directly backward. It starts on the square directly in front of the King.
When entering, moving within, or leaving the promotion zone (the three farthest ranks), a Drunk Elephant may be promoted to a Crown Prince. The Crown Prince moves one square in any direction, just like a King.
The Crown Prince is a second royal piece: a player who has one is not lost when the King is taken, and both must be captured to win. That is why a Drunk Elephant is worth promoting even when the King is safe.
As in Shogi. Pieces promote on entering, leaving or moving within the last three ranks; a Pawn, Lance or Knight that could not move again must promote. Perpetual check is forbidden, and a position repeated four times with the same player to move is a draw.