Sorry that I wrote in German, I didn't see that it is in NL.
There is no general tag for areas that are somehow used for horses. If it is used for riding (maybe in circles or lungeing) it can be tagged leisure=pitch + sport=equestrian (preset "Horse Riding / Rodeo Arena"). If not, it may be  https://wiki.openstreetmap.org/wiki/Tag:landuse=animal_keeping or https://wiki.openstreetmap.org/wiki/Tag:meadow=pasture or https://wiki.openstreetmap.org/wiki/Tag:meadow=paddock ; there is not really an established tagging, unfortunately, AFAIK.