Grant Acedrex is a medieval chess variant dating back to the time of King Alfonso X of Castile.
The following rules are from the reconstruction given on the website of Jean-Louis Cazaux, based on work by him and Sonja Musser.
The unicornio (the illustration on the medieval codex shows a rhinoceros's head) moves like a modern knight, before continuing diagonally outward any number of squares.
The game could be sped up by using eight-sided dice to dictate which piece could move that turn, ranking them in the order: king (8), aanca (7), unicornio (6), rook (5), lion (4), crocodile (3), giraffe (2), and pawn (1).
[1] The multi-variant Chess engines Fairy-Max and Postduif can play Grant Acedrex under the XBoard or WinBoard user interface.