"But the rough, arduous life of those days required outdoor shoes of leather, almost in the modern style. These were made of well-oiled zebro hide or calfskin; those of better quality were of deerskin, sheepskin, or polished goatskin." pg. 45
I can personally attest that the Sorraia horse has a tough hide, which would no doubt make for excellent work boots. The skin of Altamiro and his half-Sorraia offspring, is different than that of our domestic horses and also different than that of our draft mules and the density of their hair coat (Zorita and Altamiro, especially) is also noteworthy...its incredibly short and thick. In summer it is sleek, yet maintains an incredibly density. The skin ripples more underneath the hair and sometimes even appears to sag a bit around the eyes (more noticeable in the youngsters) and the overall sense reminds me of what I imagine an elephants skin would be like.
The most telling piece of evidence I have, however, for the resiliency of the Sorraia hide is frequently demonstrated by the "hard knocks" Altamiro can take from the heels of his pasture mates. Mistral, our Arabian gelding, is also one who rarely backs away from being kicked and has weathered some pretty severe hard knocks himself...but he always sports a crescent shaped hairless scrape or cut after such encounters. With Altamiro, he always comes away completely unscathed.
See for yourself in this bit of video footage taken last week during one of Altamiro's sneaking away from his mares and foals to come and rough house with the other herd. In this instance he and our draft mule Jerry are "horsin' around":
I'm taking this as another bit of evidence that the Sorraia is indeed the Zebro of antiquity. ;-)