Known Knight-class Units and Their Sides
Knights appear to be advanced (elite) versions of other troops. Stanley the Tool's Knights were drawn from all the (Hobgobwin) regiments in his (former) empire. Transylvito's Skanks are just a special class of Guard Goyles. It is possible that all Knights are promoted from lower classes, although it may be possible to Pop them as Knights.
Perhaps Knights are always able to ride Mounts (thus the name Knight). If so, they must always be advanced forms of non-Heavy Units, and must not become Heavy Units when they become Knights. If so, then there would be no such thing as a Twoll Knight, for instance.