Rashi on Leviticus 15:9/רש"י על ויקרא ט״ו:ט׳

1וכל המרכב AND ANYTHING FOR RIDING — even though he has not sat upon it when riding, e. g. the hold in front of the saddle, which is called arcon in old French, becomes unclean as a part of something for riding on (משום מרכב), and the אוכף, saddle, which is called alves in old French, is unclean as being something to sit upon (משום מושב) (Eruvin 27a).