Seismosaurus (CR 12)

Colossal Animal
Alignment: Always neutral
Initiative: -1 (Dex); Senses: darkvision 60 ft., low-light vision, scent, Listen +22, and Spot +21

AC: 11 (-8 size, -1 Dex, +10 natural), touch 1, flat-footed 11
Hit Dice: 32d8+300 (444 hp)
Fort +27, Ref +17, Will +12
Speed: 20 ft.
Space: 120 ft./25 ft.
Base Attack +24; Grapple +37
Attack: Tail slap +31 melee
Full Attack: Tail slap +31 melee
Damage: Tail slap 4d10+22
Special Attacks/Actions: Trample 10d10+22
Abilities: Str 40, Dex 9, Con 29, Int 1, Wis 14, Cha 14
Special Qualities:
Feats: Alertness; Awesome Blow; Diehard; Endurance; Improved Bull Rush; Power Attack; Run; Toughness x4
Skills: Listen +22 and Spot +21
Advancement: 33-64 HD (Colossal)
Climate/Terrain: Warm forests, plains, hills, and marsh
Organization: Solitary, pair, or pod (5-20)
Treasure/Possessions: None

Source: Monster Manual II

Trample (Ex): As a standard action during its turn each round, seismosaurus can trample opponents at least one size category smaller than itself. This attack deals 10d10+22 points of bludgeoning damage. A trampled opponent can attempt either an attack of opportunity at a -4 penalty or a Reflex save (DC 41) for half damage.

Scent (Ex): A dinosaur can detect approaching enemies, sniff out hidden foes, and track by sense of smell.

Seismosaurus is simply too big to have natural enemies - it dwarfs even the likes of allosaurus and tyrannosaurus. Seismosaurus is more likely to inadvertently step on an opponent than to intentionally use its tail slap attack.