We didn't pay for any of our animals - they were surrendered to us by owners who didn't realise how much they took to keep properly
Snakes - serious burns to the underside due to heatmats and used without a thermostat, fire hazard on top of being incredibly dangerous for the animal - owner hadn't taken to the vets, took us a lot to sort and had to have a while new set up with guarded bulb and UV which they had gone without
Bearded dragon - shed stuck on toes, UVB light hadn't been replaced for 3 years (they last 12 months max before the output is seriously depleted, cost £40 every six months for us for each reptile to keep them up to standard) hadn't used appropriate calcium supplements, lots of vet bills
Skinks- one of them was well loved but enclosure wasn't right for his needs, diet also needed an overhaul
Pet Cat - well he is fine, now vaccinated, neutered and wormed and flead regularly
Various inverts - people couldn't be bothered with heat and food, live feeder insects get expensive when you are buying whole tubs for an animal that eats one a week
Dogs - both big breeds, got them from seperate owners, they were being fed breeder pack, not exercised, one seriously overweight and terrified of his own shadow
Good food and healthcare with lots of input from us to train and both are happy, well behaved boys now, left where they were the behaviour problems would have got even more severe, with us they are great family dogs
Feral cats - enough said, we feed them, trap them for once a year health care, they deal ok but don't want to be pets and might never
People just decide they like the way an animal looks and don't give a thought about what it actually takes to meet their needs. Exotics especially from my experience.