Dinosaur, Battletitan (CR 16)

Huge Animal
Alignment: Always neutral
Initiative: +4 (Improved Initiative); Senses: low-light vision, scent, Spot +20, and listen +11

AC: 35 (-2 size, +7 half-plate harding, +20 natural), touch 8, flat-footed 35
Hit Dice: 36d8+324 (486 hp)
Fort +29, Ref +22, Will +15
Speed: 60 ft. (40 ft. in half-plate barding)
Space: 15 ft./10 ft.
Base Attack +27; Grapple +51
Attack: Bite +41 melee
Full Attack: Bite +41 melee and 2 claws +39 melee and tail spike +39 melee
Damage: Bite 3d6+16, claws 2d6+8, tail spike 2d6+8
Special Attacks/Actions: Improved grab, swallow whole
Abilities: Str 42, Dex 11, Con 29, Int 2, Wis 13, Cha 16
Special Qualities:
Feats: Armor Proficiency (heavy); Armor Proficiency (light); Armor Proficiency (medium); Cleave; Improved Initiative; Improved Natural Attack (bite); Improved Natural Attack (claw); Improved Overrun; Iron Will; Lightning Reflexes; Multiattack; Power Attack; Run
Skills: Jump +23, Spot +20, and listen +11
Advancement: 37-48 HD (Huge)
Climate/Terrain: Warm plains
Organization: Solitary or battalion (4) (domesticated)
Treasure/Possessions: None

Source: Monster Manual III

Improved Grab (Ex): To use this ability, a battletitan must hit an opponent of up to one size smaller with its bite attack. It can then attempt to start a grapple as a free action without provoking attacks of opportunity. If it wins the grapple check, it establishes a hold and can try to swallow the foe the following round.

Swallow Whole (Ex): A battletitan can try to swallow a grabbed opponent of up to two sizes smaller than itself by making a successful grapple check. The swallowed creature takes 2d8+12 points of bludgeoning damage and 12 points of acid damage per round from the dinosaur's gizzard. Swallowed creatures must also make a DC 37 Fortitude saving throw each round or be overcome with nausea until their next action (the save DC is Constitution-based). While in the gizzard, the opponent is still considered grappled due to a battletitan's powerful digestive muscles. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal 30 points of damage to the gizzard (AC 18). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way our. A Huge battletitan's gizzard can hold 1 Medium, 2 Small, 8 Tiny, 32 Diminutive, or 28 Fine opponents.

A battletitan is under the control of its handler in combat; the rider gives the creature commands as it would any other mount trained for war. Warriors mounted on battletitans prefer to overrun ranks of regular infantry on their way to seeking out enemy siege engines or other powerful battlefield monsters. A rider must make a Ride check to attack while riding a battletitan, and usually requires a reach weapon to attack foes.