Terminally ill pets

We have 3 pets, the obvious one, Ben and 2 not so obvious Guinea pigs called Missy and "The slug", not sure what the real name is but I refer to him as the slug because he's one of the smooth hair types. Not many people know about the Ginns since they don't go out much, they live in the shed and are fed by Florence twice a day. They've been there for years we are starting to get worried that they will live for ever. We used to have 3 but one injured its leg which died and started to rot, eventually the rest of its body died too. When you buy a pet the shop should tell you how long the pet is going to live, if they live too long you should be able to take them back to the shop. You could select an animal that was going to live for the time required. If you just wanted a Christmas present but didn't want the hassle of looking after a pet for life you could buy a terminally ill pet that would die after a few weeks. What a perfect gift to look after a sick animal during its last few days. Would be a perfect way to introduce children to the concept of death.