"A mounted warrior may, when charging with a braced weapon, substitute his own Damage Modifier for that of his mount." (Page 104 of Mythras)
Superficially, this seems to say that you can use your own Damage Modifier instead of your mount's when you charge while mounted. I think this is confusingly worded, but actually means you can substitute your mount's Damage Modifier for your own. Even if it's not what this passage means, it is what should be the rule.
The average human has a Damage Modifier of +0 and when charging, this goes up to +1d2.
The average horse has a Damage Modifier of +1d12, and this goes up to +(1d8+1d6) when charging.
These are not aberrations in the rules, but rather almost all mounts are stronger than their possible riders, and because of the doubled bonus to Damage Modifier increases that quadrupeds receive when charging, they will almost always have a higher Damage Modifier than their riders.
The mounted warrior normally makes the attack when the combined unit charges, not the mount.
I can see no situation in which someone charging would want to substitute their Damage Modifier for their mount's, and for the situation to even make sense, the mount itself would have to be making the attack on the charge, which is a nonstandard situation itself.
Therefore, it seems sensible to assume that the preposition "for" is being used in a slightly odd way here, and what the rule is asserting is that a rider can use their mount's Damage Modifier when they charge (which makes charges that hit super powerful).
As a further piece of evidence, I ran a one-shot for Lawrence Whitaker once, and someone got charged by a mounted foe, and we used the interpretation of the rule that I'm elaborating here at that time without complaint.
I'm doing a big review of Mythras combat in preparation for a Dawnlands campaign I'm starting up with my D&D 3.5 crew come mid-May. I'm spreading the good word, getting some experienced roleplayers but newcomers to Mythras to try it out and see how they like it compared to D&D 3.5.
This line struck me because I thought I knew Mythras combat reasonably well, and yet suddenly I thought that I had misunderstood a situation that I've adjudicated many times. I'm posting this at least partly so that no one else undergoes the confusion I did.
(Also, happy Easter)