What I think I'll do is replace the standard light and heavy war horses with light and heavy horses using the warbeast template from MMII.
So:
Heavy Warhorse
Size/Type: Large Animal
Hit Dice: 4d8+16 (34 hp)
Initiative: +1
Speed: 60 ft. (12 squares)
Armor Class: 13 (-1 size, +1 Dex, +3 natural), touch 10, flat-footed 12
Base Attack/Grapple: +3/+11
Attack: Hoof +7 melee (1d6+4)
Full Attack: 2 hooves +7 melee (1d6+4)
Space/Reach: 10 ft./5 ft.
Special Attacks: —
Special Qualities: Low-light vision, scent, combative mount
Saves: Fort +8, Ref +5, Will +3
Abilities: Str 19, Dex 13, Con 18, Int 2, Wis 14, Cha 6
Skills: Listen +7*, Spot +6*
Feats: Endurance, Run, Light Armor Proficiency*, Medium Armor Proficiency*,Heavy Armor Proficiency*
Environment: Temperate plains
Organization: Domesticated
Challenge Rating: 2
Advancement: —
Level Adjustment: —
Cost: 400gp
Light Warhorse:
Size/Type: Large Animal
Hit Dice: 4d8+16 (34 hp)
Initiative: +1
Speed: 70 ft. (14 squares)
Armor Class: 13 (-1 size, +1 Dex, +3 natural), touch 10, flat-footed 12
Base Attack/Grapple: +3/+10
Attack: Hoof +6 melee (1d4+3)
Full Attack: 2 hooves +6 melee (1d4+3)
Space/Reach: 10 ft./5 ft.
Special Attacks: —
Special Qualities: Low-light vision, scent, combative mount
Saves: Fort +8, Ref +5, Will +3
Abilities: Str 17, Dex 13, Con 18, Int 2, Wis 14, Cha 6
Skills: Listen +7*, Spot +6*
Feats: Endurance, Run, Light Armor Proficiency*, Medium Armor Proficiency*,Heavy Armor Proficiency*
Environment: Temperate plains
Organization: Domesticated
Challenge Rating: 2
Advancement: —
Level Adjustment: —
Cost: 400 gp
*+1 to Listen and Spot, Armor Proficiencies, and Combative Mount, all come from Warbeast template. Combative Mount gives rider +2 to Ride checks