The Captain of the Guards is a crocodile, who leads Prince John's contingent of soldiers and who, therefore, is completely loyal and subservient to him.  He does not answer directly to the Sheriff of Nottingham, but presumably shares a similar rank and station.  Being one of Prince John's henchmen, he is also a sneaky, slimy and nefarious character.

Origins of the character: While there is no direct foundation for this particular character in the Robin Hood legends, it is certain that the character of Prince John would have had a captain of the royal guards at his disposal, and probably did in the various versions of the legend.  Whether or not such a character was ever mentioned in any version of the legends is purely speculative, however.