What a silly question -dogs don’t wear pants. They wear a fur coat. But the image and question has arguments going for both sides. At first glance, you’d think the image on the right makes more sense. That’s how we wear pants. You need a waist to keep your pants up. But there’s another argument.
Well pants are a human invention so if they were to wear them like humans it would be like the ones on the right. However, if dogs invented dog pants it would be like the ones on the left, leaving the asshole freely available for greetings and ice breakers.