The act of one individual placing a strap-on dildo on his/her head, lubricating it with Dr. Bronner's all natural menthol spearmint soap, and proceeding to penetrate his/her partner's asshole. It is preferred for the "rhino" to penetrate at a full running charge. Technically speaking, this is a modified screwnicorn.
Ooooooo-weeeeeee! Damn that spearmint rhino was refreshing. It's like my asshole just brushed it's teeth.