At one point our rabbit was nearing (but not quite) being overweight, so the vet recommended we put him on Excel Light. They were the same Excel pellets we usually gave him, only a lighter version. As he got older we swapped that to pellets aimed at senior aged rabbits.
My dog recently had surgery for bladder stones. We are currently waiting for the vet to call us back after analysing the stones to tell us what special diet she'll need. I'm quietly dreading the cost of buying $35 bags of dog food!