Do not agree to light fort. The head isn't a shield at all--it's more for movement in the water. Let me post the inox, a giant Diplocaulus I converted from World of Kong...
INOX
Large Animal (Aquatic)
Hit Dice: 7d8+14 (45 hp)
Initiative: +5
Speed: 10ft (2 squares), swim 30ft
Armor Class: 13 (-1 size, +1 Dex, +3 natural), touch 10, flat-footed 12
Base Attack/Grapple: +5/+12
Attack: Bite +8 melee (1d8+4)
Full Attack: Bite +8 melee (1d8+4)
Space/Reach: 10ft/5ft
Special Attacks: Improved grab, swallow whole
Special Qualities: Amphibious, hard-to-swallow, hydrofoil, low-light vision, scent
Saves: Fort +7, Ref +6, Will +2
Abilities: Str 16, Dex 13, Con 14, Int 1, Wis 11, Cha 2
Skills: Hide +6, Move Silently +10, Spot +4
Feats: Improved Initiative, Stealthy, Weapon Focus (bite)
Environment: Warm aquatic (freshwater)
Organization: Solitary
Challenge Rating: 4
Treasure: None
Alignment: Always neutral
Advancement: 8-14 HD (Large)
Level Adjustment: -
A fat brown lizard-like creature glides through the water, its boomerang-shaped head opening to reveal a wide jaw filled with conical teeth.
Ambush predators, inox cruise the rivers and lakes of Skull Island snapping up fish and wading birds. They converge on nesting grounds of ligocristus on muddy islands, hoping to snap up hatchling dinosaurs drinking or attempting to cross the water. Primitive amphibians, their bizarre heads make them more difficult to be swallowed whole and help them cut through fast-moving currents.
Most inox reach a length between 12 and 18 feet.
Combat
Inox strike fast and first, hoping to quickly grab and swallow prey. They are unlikely to attack a troupe of adventurers crossing their path, but if hungry enough, they may attempt to snap up a halfling or gnome falling behind.
Improved Grab (Ex): In order to use this ability, an inox must hit an opponent with its bite attack. It can then make a grapple check as a free action without provoking an attack of opportunity. If it gets a hold, it can attempt to swallow its opponent whole.
Swallow Whole (Ex): An inox can try to swallow a grabbed opponent of up to two sizes smaller by making a successful grapple check. The swallowed creature takes 2d6+4 points of bludgeoning damage and 8 points of acid damage per round from the inox’s gizzard. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal 15 points of damage to the gizzard (AC 10). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.
A Large inox’s gizzard can hold 1 Small, 4 Tiny, 8 Diminutive or 32 Fine creatures.
Hard to Swallow (Ex): An inox counts as one size category larger for the purposes of being swallowed whole.
Hydrofoil (Ex): An inox gains a +4 bonus on any Swim check made to swim against a current.
Skills: An inox gains a +4 racial bonus on all Hide and Move Silently checks. An inox has a +8 racial bonus on any Swim checks to perform some special action or avoid a hazard. It can always take 10 on Swim checks even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.