Bison (CR 2)

Large Animal
Alignment: Always neutral
Initiative: +0; Senses: low-light vision, scent, Listen +7, and Spot +5

AC: 13 (-1 size, +4 natural), touch 9, flat-footed 13
Hit Dice: 5d8+15 (37 hp)
Fort +7, Ref +4, Will +1
Speed: 40 ft.
Space: 10 ft./5 ft.
Base Attack +3; Grapple +13
Attack: Gore +8 melee
Full Attack: Gore +8 melee
Damage: Gore 1d8+9
Special Attacks/Actions: Stampede
Abilities: Str 18, Dex 10, Con 16, Int 2, Wis 11, Cha 4
Special Qualities:
Feats: Alertness; Endurance
Skills: Listen +7 and Spot +5
Advancement: 6-7 HD (Large)
Climate/Terrain: Temperate plains
Organization: Solitary or herd (6-30)
Treasure/Possessions: none

Source: Monster Manual

Stampede (Ex): A frightened herd of bison flees as a group in a random direction (but always away from the perceived source of danger). They literally run over anything of size Large or smaller that gets in their way, dealing 1d12 points of damage for each five bison in the herd. A successful Reflex save (DC 16), halves the damage.