When was the horse re-introduced to the Americas?

There is a common answer here; perhaps more interesting is the trick answer that hinges on an abuse of the term "the Americas", in which case, given that roughly half of Iceland (and all of Greenland) are part of the North American plate, and thus part of "the Americas", the horse was re-introduced a bit earlier than the 15th or so century.

This is perhaps why some go to much trouble to define more exactly what terms like "the Americas" actually mean.