Feral

A feral organism is one that has escaped from domestication and returned, partly or wholly, to a wild state. The introduction of feral animals or plants to their non-native regions, like any introduced species, can disrupt ecosystems and may, in some cases, contribute to extinction of indigenous species. However, returning lost species to their environment can have the opposite effect, bringing damaged ecosystems back into balance.

Reference: http://en.wikipedia.org/wiki/Feral