Japanese Knotweed
This one is very interesting! The Japanese Knotweed was first introduced from Asia to Europe in the mid-19th century. It was viewed as a mere ornamental and fodder plant. It actually grows very quickly and prevents native seeds from germinating. Plus, it thrives on being uprooted! It can regrow anywhere after being root out.
Giant African Snail
The Giant African Snail was first introduced to the Pacific and Indian oceans as a food source for humans. Turns out it has quite the appetite and attacks all sorts of things including more than 500 kinds of plants.