Larry J. Elmore
Bozeman, Montana
http://www.trends.ca/~yuku/tran/xhorse.htm
"Now it is known that the Vikings used horses and brought them to Iceland and Greenland. They are relatively small, varicolored ponies. They also released them to run wild. They captured new horses from the wild herds and also hunted and ate them at need. The Vikings are known to have had a settlement in Newfoundland. It would be ridiculous to presume that they didn't also visit the mainland, only a few hours sail away (though apparently some anthropologists hold to that view). This would explain both the physical characteristics of the Indian ponies and give the horses 500 more years to breed and spread across the continent, and to be domesticated and used by the Plains Indians."
The possibility that they might have introduced a stable population of horses that survived them is a lot more dubious. The environment and climate of northeast America at the time was either frozen tundra or heavily forested, neither of which are environs suitable for exploting horses. This is especially true at the beginning of the second millenium when the northern hemisphere was entering a miniature ice age (which was what eventually wiped out the Norse population in Greenland).
Norse horses would have been of little use to the native Dorsett and Eskimo populations except as a short lived bonus source of meat. The chance that a small abandoned population could have survived the predations of both humans and animals to travel throught the forests to the inland plains where their natural gifts could be best exploited by the plains natives is infinitesimally small. Moreover, the amazing potential of the horse, upon arrival in the plains, means it would have been rapidly adopted and utilized by the population there as soon as it arrived. Which they did when the horse arrived from Spain, many centuries after the Norse left.