You charge the enemy and gore it with your horns.

Encounter
Standard Action      Melee 1

Effect: You charge and make the following attack in place of a melee basic attack.

  Target: One creature

  Attack: Strength + 4 vs. AC, Constitution + 4 vs. AC, or Dexterity + 4 vs. AC.
Increase the bonus to attack rolls to +6 at 11th level and +8 at 21st level.

  Hit: 1d6 + Strength modifier, Constitution modifier, or Dexterity modifier damage, and you knock the target prone.

  Level 11: 2d6 + Strength modifier, Constitution modifier, or Dexterity modifier damage.

  Level 21: 3d6 + Strength modifier, Constitution modifier, or Dexterity modifier damage.

Published in Player's Handbook 3, page(s) 10.