Pachycephalosaurus (CR 6)

Large Animal
Alignment: Always neutral
Initiative: -1 (Dex); Senses: low-light vision, scent, and Spot +10

AC: 18 (-1 size, -1 Dex, +10 natural), touch 8, flat-footed 18
Hit Dice: 7d8+56 (87 hp)
Fort +13, Ref +4, Will +2
Speed: 40 ft.
Space: 10 ft./5 ft.
Base Attack +5; Grapple +16
Attack: Head butt +11 melee
Full Attack: Head butt +11 melee
Damage: Head butt 1d6+10
Special Attacks/Actions: Powerful charge, stun
Abilities: Str 25, Dex 8, Con 26, Int 2, Wis 10, Cha 14
Special Qualities:
Feats: Improved Bull Rush; Improved Overrun; Power Attack
Skills: Spot +10
Advancement: 8-14 HD (Large), 15-21 HD (Huge)
Climate/Terrain: Temperate hills
Organization: Solitary, pair, or pack (4-12)
Treasure/Possessions: None

Source: Dragon #318

Powerful Charge (Ex): When a pachycephalosaurus charges, its head butt attack deals 2d6+20 points of bludgeoning damage.

Stun (Ex): A creature that is critically hit by a pachycephalosaurus must make a successful DC 20 Fortitude save or be stunned for 1d4 rounds. This save DC is Strength-based.

A pachycephalosaurus incited to attack (often by something as minor as merely approaching within charging range) invariably starts combat with a charge. They usually attack once every other round, maneuvering to set up a new charge on the off rounds.