Here's a start based on the monitor lizard:
Goanna, Mythic
Medium Magical Beast
Hit Dice: 3d10+9 (25 hp)
Initiative: +2
Speed: 30 ft. (6 squares), swim 30 ft.
Armor Class: 15 (+2 Dex, +3 natural), touch 12, flat-footed 13
Base Attack/Grapple: +3/+6
Attack: Bite +6 melee (1d8+4)
Full Attack: Bite +6 melee (1d8+4)
Space/Reach: 5 ft./5 ft.
Special Attacks: x
Special Qualities: Low-light vision
Saves: Fort +8, Ref +5, Will +2
Abilities: Str 17, Dex 15, Con 17, Int 1, Wis 12, Cha 2
Skills: Climb +7, Hide +6*, Listen +4, Move Silently +6, Spot +4, Swim +11
Feats: Alertness, Great Fortitude
Environment: Warm forests
Organization: Solitary
Challenge Rating: x
Treasure: None
Alignment: Always neutral
Advancement: 4–5 HD (Medium)
Level Adjustment: --
Skills: A monitor lizard has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line. Monitor lizards have a +4 racial bonus on Hide and Move Silently checks. *In forested or overgrown areas, the Hide bonus improves to +8.