Like many pet owners, Zoe and her husband feed their cat a scoop of dry food every day. But unlike most pet owners, the kibble they give their kitty isn’t made of meat—instead, it’s entirely plant-based. “I kept very quiet about it at first,” she says. You just never know how people are going to react to your dietary choices, even (or maybe especially) when those choices involve what you feed your pet. Zoe, who requested to be identified by first name only to avoid online harassment, is part of a small but growing number of cat owners opting to...