TO TRICK-OR-TREAT with her family this Halloween, Natalie Sanderson donned a fishtail braid, headset, shimmery chartreuse animal-print leggings and a tank top emblazoned with the word “Peloton,” the fitness brand. To any casual onlooker in her Portland, Ore., neighborhood, the 37-year-old resembled a particularly motivational mermaid. To those in the know, however, Ms. Sanderson was clearly dressed as the beloved Peloton instructor Robin Arzón.
Ms….