Centaur Trample( Races of the Wild, p. 148)
You have trained to use your large body and unique physiology against your foes. Much like a humanoid knight mounted on a warhorse, you have learned how to knock down opponents and ride over them in combat.
Centaur, Dex 15,
When you attempt to overrun an opponent, your target may not choose to avoid you. You may make one hoof attack against any target you knock down (remember that prone targets take a -4 penalty to Armor Class). See Overrun, page 157 of the Player's Handbook.
A centaur fighter may select Centaur Trample as a bonus feat. The DM may make this feat available to other centaurlike races if they are available in your campaign.