Disposable pets are those small, furry or feathered things which seem to come and go frequently, particularly in families with several children. They have a rather high mortality rate - particularly when they are purchased as part of a "responsibility-development" program for a small child. Often, they can um, pass on and be replaced the same day, and no one is the wiser.
Hamsters, parakeets, mice, little turtles from the pet store, and the more common fish for fishbowls rather than large tanks - like goldfish, mollies, etc. - are all examples of disposable pets. They have to be rather generic in color and pattern, the more easily to be replaced; after all, who can tell one mollie from another? But a dog or cat is easily identified, and therefore NOT a disposable pet.