Runners in various athletic garb, as well as one individual in a spotted-green dragon costume, run to the right.
Runners see a dragon costumed person
Runners and individual with dragon costume run to right implies that runners see the dragon costumed person.
Runners in various athletic garb are types of runners, and spotted-green dragon costume is a rephrasing of dragon costumed person.
They see a dragon costumed person because there is an individual in a spotted-green dragon costume.