Europe's era of colonization was accelerated by the diseases it's explorers carried such as smallpox, killing off (unfortunately & sadly) the native inhabitants in their path who might have brought resistance to colonial expansion. Others have considered what could have happened had the disease vectors been the other way, if Native peoples had happened to have diseases that affected Europeans & a new plague been brought back on ocean going sailing vessels.
In this thread I'd like to consider the possibility of a natural animal pest having been stumbled upon & being brought back. There might be small natural examples in OTL. More specifically however given the worst aspects of the animals we know now to have been discovered or which have existed in recent pre-history, what biologically plausible creature could have existed that might have embodied them all & survived a ship voyage back? What effects might it have had?
An insect that lays it's eggs in humans for maturation? One whose lifestages mean it's nearly impossible to eradicate? That eats crops in swarms/kills livestock with it's sting?