As someone into Japanese history, I always wondered something- why didn’t the Japanese make more extensive use of horses during their wars? I knew they made some use of them, but nowhere near as much as people from other countries did.
What I didn’t expect to find was the reason why Horses weren’t used much in Japan by the Samurai the way they were in many other parts of the world. They used them, but only in fairly small numbers, and I’d always wondered why. Well apparently the answer is that Japanese native horses are actually pretty small.
This meant that they had a very limited ability to carry a Japanese Samurai (much less one in full armour) for long distances and thus were apparently only used by commanders and messengers in war. The Japanese apparently didn’t even bother to have actual mounted cavalry units per-se.
Here are pictures of Samurai and horses for comparison-
The last picture (painting, really) shows a clear-ish view of what a saddle of the period looked like too. (And this is likely the style of saddle Masato would be using in the story.)