I suppose you could add HD to the creature as well per the Monster Advancement rules, if you wanted to make a particular tough heavy warhorse.
That is what I was suggesting. A master work equipment: well fed, well worked, well trained, good breeding. Unfortunately, it takes a horse to the age of about 2 1/2 years old to be ridden: start riding at two, about 6 months to saddle train.
Advanced heavy horse
N Large animal
Init +4; Senses low-light vision, scent; Perception +8
Defense
AC 15, touch 13, flat-footed 11 (+4 Dex, –1 size, natural +2)
hp 19 (2d8+10)
Fort +8, Ref +7, Will +3
Offense
Speed 50 ft.
Melee 2 hooves +6 (1d6+5); bite +6 1d4 +5
Space 10 ft.; Reach 5 ft.
Statistics
Str 20, Dex 18, Con 21, Int 6, Wis 17, Cha 11
Base Atk +1; CMB +7; CMD 19 (23 vs. trip)
Feats Endurance, RunB
Skills Perception +8
SQ
Ecology
Environment temperate plains
Organization solitary, pair, or herd (3–30)
Treasure none