Dimetrodon (CR 2)

Large Animal
Alignment: Always neutral
Initiative: -2 (Dex); Senses: low-light vision, scent, Listen +2, and Spot +6

AC: 13 (-2 Dex, +5 natural), touch 8, flat-footed 13
Hit Dice: 4d8+15 (33 hp)
Fort +7, Ref +2, Will +1
Speed: 10 ft., swim 10 ft.
Space: 5 ft./5 ft.
Base Attack +3; Grapple +7
Attack: Bite +7 melee
Full Attack: Bite +7 melee
Damage: Bite 1d8+6
Special Attacks/Actions: Burst of speed
Abilities: Str 18, Dex 6, Con 17, Int 1, Wis 10, Cha 10
Special Qualities:
Feats: Alertness; Toughness
Skills: Listen +2, Spot +6, and Swim +15
Advancement: 5-6 HD (Medium), 7-12 HD (Large)
Climate/Terrain: Warm marsh
Organization: Solitary, pair, or pack (3-7)
Treasure/Possessions: None

Source: Dragon #318

Burst of Speed (Ex): Up to three times per hour, a dimetrodon can increase its land speed to 60 feet for 1 round as a free action. During this round, it gems a +2 dodge bonus to its Armor Class.

Although it often remains motionless for long periods of time, a dimetrodon is highly territorial and reacts to intrusions with sudden and terrifying speed,