In the ancient Indian game Chaturanga, what rules are known, and how does it vary from modern chess? Also, is it a solved game?
The exact rules aren't known, but what is known or commonly believed to be true is listed on Wikipedia. Especially the elephant/gaja's movement rules are uncertain.
It varies from modern chess because it has some pieces which move differently: the elephant mentioned above and the mantri/minister. Modern chess has the bishop and queen in their places. Still, the complexity of the game is comparable to chess, so it's a long way from being solved.