It's not as if this hasn't happened before. The first animals to walk on land evolved from fish, probably four or five hundred MILLION years ago.
There's also a 'walking catfish' (Clarias batrachus), from Asia, that was or is a big problem in the Southern United States; I read about it in a National Geographic from about 50 years ago. People imported them illegally as tropical fish, then let them loose in ponds, or flushed them down the toilet, or something like that. Their natural habitat was ponds and small lakes that dried up at certain times of the year, so they 'walked' to other ponds that still had some water in them. There wasn't much oxygen in a pond formed by monsoon rains, after the first month or two of the dry season.
If that interests you, search for walking catfish on wikipedia.