Advancing a giant crocodile to 25 HD and Gargantuan, and switching to magical beast yields...
Gargantuan Magical Beast
Hit Dice: 25d10+150 (287 hp)
Initiative: +1
Speed: 20 ft. (4 squares), swim 50 ft.
Armor Class: 18 (–4 size, +1 Dex, +11 natural), touch 7, flat-footed 17
Base Attack/Grapple: +25/+49
Attack: Bite +33 melee (3d8+18) or tail slap +33 melee (3d6+18)
Full Attack: Bite +33 melee (3d8+18) or tail slap +33 melee (3d6+18)
Space/Reach: 20 ft./15 ft.
Special Attacks: Improved grab
Special Qualities: Hold breath, low-light vision
Saves: Fort +20, Ref +15, Will +9
Abilities: Str 35, Dex 12, Con 23, Int 1, Wis 12, Cha 2
Skills: 28
Feats: 9 (2 can be epic)
Environment: Warm marshes
Organization: Solitary or colony (6–11)
Challenge Rating: x
Treasure: None
Alignment: Always neutral
Advancement: x
Level Adjustment: -
Look like a good start?
We'll need to add claw attack(s) and possibly darkvision (since it usually comes with the magical beast type).