Mustang is around 16 years old. He wears his maturity well. His health is pretty good, but like many of us, he could stand to lose a few pounds. The caregivers at Cat World tried to put Mustang on a diet, but it didn't go so well. Mustang loves other cats — in particular, the group he lives with. He prefers their company over that of most humans. So when the caregivers moved Mustang to cat headquarters so that he could be in a “private” room where his feedings could be monitored, his tender heart broke.
His exile (and diet) lasted 24 hours. He was simply too distressed and his mournful yowling broke everyone's heart. It became clear that the real threat to his health wasn't his muffin top. It was the distress and anxiety in this new situation. So he returned to his friends with apologies, and probably a snack.