If you are into informal English style horse riding then wearing jeans wouldn't be a problem. However, make sure that you pair them with long boots. This will prevent your calves from being pinched by the leather on the stirrups.  Also make sure that whichever jeans you wear, they have a flat seam so that you are comfortably seated on the saddle. You can also opt for denim breeches. They give the same feel as jeans and do not have an inseam, too. Nevertheless, plain jeans would do, too. Must read: Horse Riding Styles: The Complete Guide.