In the beginning you state that by "guard" you mean a piece that moves as a King. I think it's incorrect to cite pieces like the Shi in Xiangqi or the Dabbabah in Courier as examples, since they move differently (the Shi may be called "Guard", but it is a Ferz. The Dabbabah isn't even called a guard).
However, a non-royal King is an interesting piece. It has mating potential against a lone king, and it can draw against a Queen (if it can be defended by the defending King there is no way for the Queen to force them apart).
Spartan Chess effectively has this piece (it has two kings, but the special checking moves in that variant practically makes them non-royal while they are both on the board). In that game its value is higher, just below a Rook at 4.5. This is at least partly due to the increase in king safety it provides.
by vickalan
The guard is a somewhat misunderstood, and critically endangered chess piece. He's not used in classical chess. In the games where you can find him, he usually moves and captures the same as King - but he is not a royal piece, and has no royal powers. Although the King and guard move the same, victory depends upon the fate of the king - but the guard can be captured with no penalty other than the loss of the guard himself.
(2) The guard in Courier chess or Quatrochess (inverted perhaps because it is not a royal piece?)
(3) Shi in Old Chinese Chess (Xiangqi)
(4) Huscarl in Medieval Chess
(5) The guard in Mini Citadel Chess
(6) The guard in a Custom Chess Variant
(7) The guard in Waterloo
(8) The guard in Waterloo using a hieroglyphic
(9) Guards (at ends) protecting the King and General in Shatranj Kamil (as Dabbabah)
(11) The guard in Courier-Spiel chess (as Fool)
(12) Guard/Prince used in chess variant (moves as king)
(13) Prince (moves as a king) used in chess variant
(14) Guard in Exotic Chess
(15) The guard in Bulldog Chess (classic style)
(16) The guard in Bulldog Chess (gothic style)
Few chess engines have a guard programmed in their play. And available endgame tablebases for six or seven pieces include classical chess pieces, but not a guard. So these gaming tools provide little help to players seeking to optimize their play with these resources.
The guard is not the strongest piece, but he is not a weak piece either. He excels at attacking and defending pieces and pawns in close proximity. Wikipedia's article on the guard ("Mann" in chess) says he is "approximately equal in strength and value to a knight" but "takes a few moves to get the mann properly developed in the opening".
A guard often, but not always moves and captures like a king. In Waterloo, the guard is stronger than the king - he can jump one or two squares orthogonally or diagonally in any direction.
Notable for its complicated movement is the guard in Divergent Chess. To quote directly from the Chess Variant Pages (20), "A Guard moves without capturing one square diagonally then optionally one square orthogonally outward. It does not capture when moving in this way. A Guard captures by moving one square orthogonally then optionally one square diagonally outward. It may only capture when moving in this way. It may not jump over pieces along the way. Once promoted on the last row, it is able to leap like an orthodox Knight and move like an orthodox King."
It's unclear if the guard will ever become more popular in chess. If simply out of tradition or the desire to maintain orthodox principles, he is likely to remain a reclusive and uncommon piece. That's not to say that the lineage of variant-chess pieces has ended or that new variants aren't being explored. On the contrary, variant chess seems to be alive and well. But many of the newest pieces found in chess seem to be fantastical new pieces with powerful and unique abilities.
To the author's knowledge there is no commercially available physical chess piece designed exclusively to represent a guard. For the player who includes a guard in OTB play, the "fortress" from the "Musketeer" chess set has been suggested as a piece which can represent the guard (image below). Unused pawns or other pieces can also be marked or modified to indicate the piece as a guard.
To players willing to understand the guard's unique defensive and offensive abilities variant chess battles can be enriching and rewarding. The guard is loyal to his King and can bring victory to his army. The player should understand a guard takes a few moves to get positioned in the opening, and travels slowly to his battles. But unless at the edge or corner of the board, the guard always controls eight squares. And although limited in range, he fights in all directions. He sharpens the game, and coordinates his powers with other pieces to create new and interesting mating patterns.
game: Bulldog chess
white: "vickalan"
black: "astuteoak"
result: "0-1"
A sample game (LXIVC vs. IvanKosintsev) is on this forum: (link)
A sample game (EvertVB vs. vickalan) is on this forum: (link)
1 - 'http://www.chessvariants.com/large.dir/contest/divergentchess.html
2 - (Wikipedia)
3 - 'http://primaltrek.com/xiangqi.html
4 - 'http://www.medievalchess.com/about.html
5 - 'http://www.frontiernet.net/~alcove/chess/largecontestindex1.html
6 - 'https://www.chess.com/forum/view/general/custom-chess-variants
7 - 'http://chess-checkers-go.blogspot.com/2014/08/waterloo.html
8 - 'http://chess-checkers-go.blogspot.com/2014/08/waterloo.html
9 - 'http://www.chessvariants.com/historic.dir/shkam2.html
10 - 'http://www.chessvariants.com/piececlopedia.dir/man.html
11 - 'http://www.chessvariants.com/piececlopedia.dir/man.html
12 - 'http://www.chessvariants.com/graphics.dir/alfaerie/
13 - 'http://www.chessvariants.com/graphics.dir/cazaux/catalog.html
14 - 'http://www.pathguy.com/chess/ExoticCh.htm
15 - 'https://www.chess.com/forum/view/chess960-chess-variants/bull-dog-chess-theory-comments-etc
16 - 'https://www.chess.com/forum/view/chess960-chess-variants/bull-dog-chess-theory-comments-etc
19 - 'http://www.chess.com/article/view/chess-piece-value
20 - 'http://www.chessvariants.com/large.dir/contest/divergentchess.html