Bad Outcome 1: Doing Too Well
If you release your pet into the wild, it may to "too well." Your hamster to find a mousey-mate and populate your neighborhood, leading to serious ecological consequences and health risks for your neighborhood. By releasing a pet into the wild, you may unintentionally introduce an invasive species, which has the potential to wreak havoc on the surrounding ecosystem.
Released pet iguanas, pythons and parakeets have caused serious harm in the Everglades, where they have displaced many native species. Red eared slider turtles have overtaken hundreds of lakes where they are not native; in the process, they have caused the local extinction of several fish and reptile species. Asian arp, guppies and goldfish also upset the balance of river ecosystems throughout the U.S.
If you care about animals, you should be aware that your pet's "success" in the wild would come at the expense of native species. Your gerbils may displace local shrews. Your rabbit can dilute the genepool of wild cottontails. Your clawed frog may eat the fry of endangered fish in local streams. Non-native animals can not live as part of a balanced ecosystem outside of their natural range.
Bad Outcome 2: Death
If your pet does not do unusually well in the wild, the alternative is a painful death. Exotic species tend to react in one of two ways to a new environment-- they spread like wildfire or they die instantly. Your pet is adapted to live in its own native environment, or, in the case of domesticated species, it has been carefully bred to live only under the care of humans. If your pet doesn't thrive, he will probably die within a matter of just a few days.
Unless you live in a subtropical region, tropical exotic pets such as iguanas and parrots will die quickly in the wild. If the animal survives into the winter, there will be no food for it and it will die of hypothermia. Your pet may also be unaccustomed to foraging; a pet who eats only commercial feed may have no ability to find food in a barren landscape. For this reason, many organizations consider it to be an act of animal cruelty to release a pet into the wild.
If you can't take care of your pet anymore, get to work on finding a more suitable home for it. Contact your local humane society, veterinarian and area pet stores for assistance in re-homing your animals. It is never appropriate to release pets into the wild.