Ok I'm just shooting out an idea here since no one else has serenaded so far.

I do not have much experience with the <calculate> tag  but the extra damage might work with a on meleespell that sets the passed tp the defensible damage tag as unit damage + bonus * target_unit_is_mounted

as well you might get lucky by introducing a unit stat  cavalry_counter and then on defend  use a prereq on the calculated value of  Unit_cavalry_counter * target_is_mounted and setting  Unit_cavalry_counter = 0 for 1 round

this is just a rough idea I had while your question was lingering in the corner of my mind. If you succeed in implementing it please come back and tell us about your struggle. I would like to learn from it

What about a damage revenge for attacking a spear holder with a mounted unit?

So will only do damage to someone that attacks it only if its a mounted unit.  This might be more what I am going for, though I do like the idea of having them do naturally more damage to mounted units I want a revenge damage for mounted units and originally only thought of counterattack but maybe a damage shield effect only for mounted the tag.

You'll have to test the counter-attack thing. It could work, or maybe it doesn't. Don't know - counter-attack is handled a bit differently because it needs to save how many has been done each round.

Alright I decided to mess around with this a bit tonight and for some reason its not working the way I wanted to.

I started with a base off of the cloak of thorns spell as its what I actually wanted not what I originally stated, sorry for the horrible clarity.

Does this look right?

Not vsunittype, vsHigher. It's a dynamic unitstat, not <creaturetype>

Also note that it won't function if the pike unit is mounted.

If the damagereflect and the value are right, I don't know that unitstat.

i have mod in the works that add alot of "new" weapons (copy and pasted hero weapons with different stats) .. i have cavalry only weapons like lances and cavalry sword that do bonus vs unmounted ... and i have spear do less armor ignore (like 34%), instead they have like +4 atk vs mounted and like +10 crit vs mounted, theres different amounts as u advance techs... and i have special spears unlocked at war colleges that let the unit do ability : spear wall ... the unit takes defense stance (like skips turn) but gains +3 counterattacks and +3 atk vs mounted for 3 turns (the weapon already has like +4 atk vs mounted, +16 crit vs mounted, +7 accuracy vs mounted (12 base damage) ) ... but thats just at war colleges, weapons of war will have higher stats, and new tech "college drills" will have another...

there will be some special anti cavalry weapons like that, because i added a tech after war colleges "cavalry tactics" which adds warhorse that give different abilitys ... harrassers warhorse has abilitys that make enemy skip a turn, or have less move range, and like berserker warhorse berserks, and defender warhorse can make ur army flee, but have the cav unit lose health... and tactical warhorse has ability to attack all adjacent enemies, for like 70% normal damage...

...

and much more....  coming soon heheh