Horses went extinct in North America and did not return until they were ferried in boats by European settlers (no land mammal can swim on its own across an ocean). From a horse's perspective, the ocean barrier is best described as what

sweepstake route

Explanation:

From the view of the horses, the ocean barrier would be seen as a  sweepstake route.

By definition, the sweepstake route refers to a rather difficult or dangerous path through which animals can move from one place to another. No animal would take this path ordinarily except by pure chance or when events leave no other option.

In the illustration, no horse would engage in an adventure of swimming across the ocean to North America until the European settlers decided to ferry them across. Hence, to the horses, the ocean barrier represents a sweepstake route.

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