Complex sentence: It is an overall rating of a player's horsemanship, team play, knowledge of the game, strategy and horses, and at one time polo was the only sport in the world that considered sportsmanship when rating a player.
Split into simpler sentences: It is an overall rating of a player's horsemanship, team play, knowledge of the game, strategy and horses. At one time, polo was the only sport in the world that considered sportsmanship when rating a player.