A feral animal is an animal living in the wild but descended from domesticated individuals.
As with an introduced species, the introduction of feral animals or plants to non-native regions may disrupt ecosystems and has, in some cases, contributed to extinction of indigenous species.
According to dictionary definitions a feral animal is one that has itself escaped from a domestic or captive status and is living more or less as a wild animal, or one that is descended from such animals. Other definitions define a feral animal as one that has changed from being domesticated to being wild, natural, or untamed. Some common examples of animals with feral populations are horses, dogs, goats, cats, and pigs.
Zoologists generally exclude from the feral category animals that were genuinely wild before they escaped from captivity: neither lions escaped from a zoo nor the sea eagles recently re-introduced into the UK are regarded as feral. Wild species naturalized into a new territory are not normally considered feral animals.
(source : wikipedia)