If you plan to use ssurrans of if your PC are going in the area of the Scorched Plateau, here is my attempt for the stats of the Fortress Beetle :
Hit Dice: 10d8+40 (85 hp)
Speed: 30 feet (6 squares)
Armor Class: 21 (-2 size, +13 natural), touch 8, flat-footed 21
Base Attack/Grapple : +7/+24
Attack: Bite (+14 melee, 2d6+13)
Full Attack: Bite (+14 melee, 2d6+13)
Space/Reach: 15 feet/10 feet
Special Attacks: —
Special Qualities: Darkvision 60 feet, vermin traits
Saves: Fort +11, Ref +2, Will +3
Abilities: Str 28, Dex 8, Con 18, Int —, Wis 10, Cha 9
Environment: Scorched Plateau, stony barrens
Organization: Solitary, pair, cluster (2-5)
Challenge Rating: 4
Advancement: 11-12 HD (Huge), 13-14 HD (Gargantuan)
Level Adjustment: —
Fortress beetles grow to a length of 25 feet, carrying sturdy, 15-foot-tall protective shells atop their backs. Much of the upper portion of these shells is hollow, with intricate and beautiful chambers formed during the growing process. These beetles are unable to fly, and it is the involution of their wings which is at the origin of the cavities present under their elytra. Access to the chambers requires lifting the elytra of the fortress beetle.
Ssurran use these docile insects as beasts of burden, filling their hollow shells with their most precious trade goods.
The load that can be carried by fortress beetles is as follows: light load up to 2.400 pounds, medium load between 2.400 and 4.800 pounds, heavy load beyond 4.800 pounds.