here's demiurge's version, just to keep discussion alive. i'll post the earlier stats when i get a moment of free time in which to do it.
Anatotitan
Huge Animal
Hit Dice: 12d8+90 (144 hp)
Initiative: -1
Speed: 40ft (8 squares)
Armor Class: 13 (-2 size, -1 Dex, +6 natural), touch 7, flat-footed 13
Base Attack/Grapple: +9/+25
Attack: Tail slam +15 melee (1d8+12)
Full Attack: Tail slam +15 melee (1d8+12)
Space/Reach: 15ft/10ft
Special Attacks: Trample 2d10+12
Special Qualities: Low-light vision, scent
Saves: Fort +15, Ref +7, Will +5
Abilities: Str 26, Dex 9, Con 24, Int 1, Wis 12, Cha 7
Skills: Listen +8, Spot +8, Swim +13
Feats: Alertness, Endurance, Run, Toughness (x2)
Environment: Temperate and cold plains
Organization: Solitary, pair or herd (6-36)
Challenge Rating: 7
Treasure: None
Alignment: Always neutral
Advancement: 13-24 HD (Huge), 25-36 HD (Gargantuan)
Level Adjustment: -
This massive creature has a leathery hide, long slender limbs and a wide beak somewhat like that of a duck.
Anatotitans are huge herbivorous ornithischians that are generally harmless unless provoked.
The average anatotitan is 30 to 40 feet long and can weigh up to 3 tons. Despite its bulk, it is shy and not aggressive. Anatotitans, like other hadrosaurs, rear their young and have large herds consisting of one or more families. They are quite resistant to cold temperatures, despite their reptilian appearance.
Combat
Anatotitans avoid combat whenever possible, preferring to flee from confrontation. If their young are threatened, however, they fight with their powerful tails.
Trample (Ex): An anatotitan can literally run over any creatures in its way that are of Large or smaller size. Creatures trampled must make a Reflex save (DC 24 half) or take 2d10+12 damage. The save DC is Strength based.