Well, Jemal is up for judging that, but I'll provide my thoughts on the subject:
Frankly, you can have ANY animal as your companion under the 3.0 rules, though you have to find one (or more), they don't appear out of thin air like the paladin's warhorse does. For most animals, you can assume you found them in the wild earlier during your life, but light and heavy warhorses, who are still animals, are not encountered in the wild, you have to get your hands on one in another way (like buying one or training one). Also keep in mind that a heavy warhorse takes up 4 HD slots, where the other horses only take up 3.