Let's see here. Here's a quick version of a mekillot, a monster from the old Dark Sun setting that was a great big lizard that was used to pull wagons and such.
(I checked
www.athas.org for the same thing, but the mekillot they had there was CR 12, and thus a bit more powerful than what you were asking for. This isn't quite as tough, even though you don't want to go making it mad at you.)
Mekillot
Huge Animal
Hit Dice: 9d8+55 (95 hp)
Initiative: -2
Speed: 30 ft.
Armor Class: 16 (+10 natural, -2 size, -2 Dex)
Base Attack/Grapple: +6/+22
Attack: Bite +12 melee (2d6+12)
Full Attack: Bite +12 melee (2d6+12)
Space/Reach: 20 ft/10 ft
Special Attacks: Trample 2d6+12 (DC 22)
Saves: Fort +14, Ref +1, Will +3
Abilities: Str 26, Dex 6, Con 22, Int 2, Wis 10, Cha 6
Skills: Listen +6, Spot +6
Feats: Diehard, Endurance, Great Fortitude, Toughness
Environment: Deserts
Organization: Solitary or herd (3-10)
Challenge Rating: 4
Alignment: Always neutral
Advancement: 10-14 HD (Huge), 15-18 HD (Gargantuan)
Level Adjustment: --
Edit: Toned down the CR a bit, since you wanted something in the 4-5 region.