they would also have been lucky I should think if all the ships they sent out managed to arrive safely.Quote
What if they transported them while they are still weaning foals?
Seems likely that while the mares might eventually die many of the foals would survive the long journey while taking up less space and creating less waste.
Then there's the space takekn up with saddles, bridles, blacksmiths and all the other things that go with using horses.
When I was reading something from Wikipedia, it pointed out that when William the conqueror crossed the Channel, they had to bring about 2,000 horses over. That must have meant that, apart from much long-term planning for the conquest, there was also much planning for the building of ships for the horses and their food, etc .