You know what? Time for some slug posting.
I was thinking today about how the guy who discovered the eastern caverns must have felt, when he first figured out how to get his slug to transform. Because from the story the series gives us, it sorta seems like he does it in the moment. He's on his horse, desperately trying to get her to move faster and faster. He draws his bow. He's facing an army with a single bow, a horse, and this tiny orange thing that's taken a liking to him.
He's gotta get moving pretty fast on the horse before he can even fire his arrow. Then he has to raise that bow, notch an arrow that has a little basket on it for this strange creature instead of a broad head, and pray to whatever gods he has left after all of this that he's making the right call. That he's put the pieces together correctly. That this phenomena he's guessing at, that maybe he's observed when slugs fling themselves off of cliffs for fun, can be replicated by a horse going at max speed when he fires his bow.
Could you imagine the desperation that drove him to this? The fear? The number of instincts and natural cautions you'd have to suppress to even attempt this?
He had to have been terrified out of his mind. One man, his horse, and a colorful slug against an army.
And against all odds, when he hits full gallop and releases his bow, it works.
Note that, in all likelihood, the horse isn't actually necessary for regular slug shooting, long bows, and Chinese equivalents, shoot at 100-130 mph, or higher depending on draw weight. However, if you had a bow with a higher draw weight you could definitely hit like, 150-170, but probably not the covered 200 mph. But..... Horses at full gallop can hit 30 mph. So it's possible the horse is needed for 200 mph slugs in this period. We know he fought the armies while riding his horse, and it adds to the dramatism for me, so realistically it doesn't matter if it's realistic(lol), as it definitely tells a better story this way. Just covering my facts for fellow weapons nerds in the audience!