Medieval guidons were triangular flags, attached to lances, which served as field signs of the medieval lance, the smallest tactical unit in the middle ages. Guidons, like pennons, repeated the tinctures and charges of the knight's shield, and they could be made long enough to show a motto.
After the middle ages, mounted units continued to carry cavalry guidons, but of a shorter, swallow-tailed design. Company-sized elements of the US-Army and United States Air Force carry swallow-tailed guidons even today to signify their unit designation and corps affiliation.
Professionally designed medieval guidons for your miniature knights may be downloaded from the Miniatures Forum.