It was not a documentary .Many of those who have heavily criticised this DRAMA seem to have forgotten this . So there are some historical inaccuracies.Most dramas have them .If you have lived through the time you have a right to point out those things you think the researchers,the writer,the producer,stage manager,costumes etc have in your view portrayed inaccurately.
As for the Icelandic ponies or horses as they prefer to call them which breed of equines could be more accurate, the Vikings had left behind their Scandinavian breeds and larger weight bearing horses were not yet needed as the armour worn by soldiers was not yet heavy enough to need it. Icelandic or Nordic "ponies" could and still can carry a grown man at speed for long distances. Their tolting gait is fast and comfortable and they are sure footed over all terrain . Inaccurate I don't think so for this particular time.
It was a drama.Sit back and enjoy it as such and have a suspension of disbelief watching some of our best actors transport us through history for a few hours.