Jetan has often been accused of unclear rules, but a careful analysis of The Chessmen of Mars shows that there are in reality very few truly unclear rules. See especially Chessmen of Mars Chapter 2, Chessmen of Mars Chapter 17 and Chessmen of Mars Chapter 22 (appendix at the end). Further clues can be found in the original pulp publication of the text.
Pieces always move their exact count of steps, not less.
Panthans can never move backwards, not even diagonally backwards.
Panthans cannot promote, and it is usually not a problem since they tend to get stuck in midfield.
Thoats do not move like chess knights, since they can also turn back to the square next to the one where they started their moves.
Warriors always move orthogonally, never diagonally.
The princess can always move through a threatened square, although she is not allowed to stop her move on one.
The chief can move any combination of orthogonal and diagonal moves, up to its three square limit.
The only serious remaining lack of clarity is whether the thoat can jump or not. Most interpretations follow Edgar Rice Burroughs's appendix where the that is not a leaper.