The list of invasive species is huge, and I'd guess it started when man started to travel the oceans long distance.
You've got plants, insects, birds, reptiles, probably micro-organisms, mammals, every sort of life-form imaginable.
Of course we only hear about the stuff invading the US, but I'm sure some US species are a problem in foreign lands, as well.
I know there are tons of plants and insects that are a huge problem, everywhere.
once you get a breeding population established, you're FUCKED !
I think rats came from Europe.
Imagine an insect that could wipe-out the crops on Fishhead's Iowa farms....maybe bio-engineer such a pest, if your goal is to take out the US farming interests.
There are no snakes in Hawaii, release enough boas, etc....they take over, and kill off every native bird, there.
I think all of this accidental, not malicious, so far, but it is huge.
Hawaii is a great example, being so isolated. Imagine the impact of some predator, like a coyote or wolf being introduced....or plant,bug,bird,etc.
How about maybe piranha in a body of fresh water ?