When and why was en passant invented? Other than castling, the only "strange" move that chess pieces can ever make is en passant.  It seems a little odd to me that someone thought that such a "different" move was so important to include in the legal moves.  When and why did this come to be?
From Wikipedia:

Allowing the en passant capture is one of the last major rule changes in European chess that occurred between 1200 and 1600, together with the introduction of the two-square first move for pawns, castling, and the unlimited range for queens and bishops (Davidson 1949:14,16,57). Spanish master Ruy López de Segura gives the rule in his 1561 book Libro de la invencion liberal y arte del juego del axedrez (Golombek 1977:108). In most places the en passant rule was adopted as soon as the rule allowing the pawn to move two squares on its first move, but it was not universally accepted until the Italian rules were changed in 1880 (Hooper &amp; Whyld 1992:124–25).
The motivation for en passant was to prevent the newly added two-square first move for pawns from allowing a pawn to evade capture by an enemy pawn. Specifically, the rule allows a pawn on a player's fifth rank the opportunity to capture the opponent's pawn on an adjacent file that advances two squares on its first move as though it had advanced only one square (Davidson 1949:16). Asian chess variants, because of their separation from European chess prior to that period, do not feature any of these moves.