I don't know much anything about them either but I did a little research for curiosity sake. The pics on those links you posted are some gorgeous horses. I really love the look of them.
Breeds it originates from today: "Today's Dutch Harness Horse may have a pedigree of many breeds, for they have steadfastly maintained an "open" studbook whereby any horse may be used, as long as it is considered and proven "acceptable" according to the high standards of the Dutch. Over the last century, select Hackney stallions have been imported from England, and most Dutch Harness Horses today possess considerable Hackney blood. More recently several American Saddlebred stallions were approved for injection of Saddlebred characteristics into their gene pool." - From REESINK HARNESS HORSES I believe that they are breeding for specific characteristics that if found in any particular breed they will incorporate it into the DHH's bloodlines. They're looking for the high stepping action, refined head and elegant neck, a specific height range usually around 16+hh, and some other things. I think it's just a hodge-podge of various elegant looking, tall, high-stepping stallions that the Dutch royalty felt suited their needs of the time period. 
