Under the act, which comes into force in England and Wales at the end of the month, it will be an offence not to meet pets' essential welfare needs, including a suitable diet.
In one advert, Daryl is approached by a friend, who wonders if he is moving to Los Angeles. It turns out that the weight loss has happened because Daryl's owner has not been around to feed him.
Andy Foxcroft, chief officer of the RSPCA's Inspectorate, said: "One dog asking another if he's 'going for the size zero look' may seem odd, but if you met a friend who was looking ill or who was being treated badly, you'd say something. We want people to think about their pet's welfare needs in the same way."