It is usually applied to ex-domesticated animals that have returned to the wild - e.g. feral cats and dogs. One could argue that domesticated cats aren't trained either! (Dogs have owners, whereas cats have staff.)
Yes indeed Chris!
The COD gives, almost exactly, your def of 'feral'.
Chambers doesn't give it at all under 'feral' but under feralised/feralized = run widl after domestication