The MM2 has a Titanic template that lets you turn any Medium or smaller animal or vermin into a Gargantuan monstruosity.
If you houserule the template a bit to allow it for more monster types, I like the Titanic Kobold.
Why? Just because!
Titanic Kobold, Gargantuan Humanoid (Reptilian)
Hit Dice: 20d8+140 (230 hp)
Initiative: +0
Speed: 30 ft. (6 squares)
Armor Class: 30 (-4 size, +20 natural, +4 mithril chain mail), touch 6, flat-footed 30
Base Attack/Grapple: +15/+27
Attack: Spear +25 melee (3d6+27/×3) or sling +17 ranged (2d6+14)
Full Attack: Spear +25/+20/+15 melee (3d6+27/×3) or sling +15/+15/+10/+5 ranged (2d6+14)
Space/Reach: 20 ft./20 ft.
Special Attacks: Trample (3d8+19)
Special Qualities: Darkvision 60 ft., light sensitivity
Saves: Fort +21, Ref +14, Will +14
Abilities: Str 36, Dex 10, Con 24, Int 11, Wis 11, Cha 11
Skills: Jump +40, Tumble +30
Feats: Acrobatics, Point-Blank Shot, Power Attack, Rapid Shot, Skill Focus—Tumble, Weapon Focus—Sling, Weapon Focus—Spear.
Environment: Temperate forests
Organization: Gang (4-9), band (10-100 plus 100% noncombatants plus 1 3rd-level half-dragon sergeant per 20 adults and 1 fiendish half-dragon leader of 4th-6th level), warband (10-24 plus 2-4 titanic dire weasels), tribe (40-400 plus 1 3rd-level axiomatic half-dragon sergeant per 20 adults, 1 or 2 pseudonatural half-dragon lieutenants of 4th or 5th level, 1 fiendish half-dragon leader of 6th-8th level, and 5-8 titanic dire weasels)
Challenge Rating: 13
Treasure: Standard
Alignment: Usually lawful evil
Advancement: By character class
Level Adjustment: +0
(I went it and fudged with PBR/Rapid Shot, Weapon Focus, Power Attack, and Skill Focus in order to only get nice round numbers. Why choosing Jump and Tumble as the two maxed skills? Because it's fun.)
