Sometimes, size does matter.

I always thought the Phantom Fungus monster was pretty lame, but it's nasty if you bulk it up some and give it regeneration. Take a look at the new and improved Huge Regenerating Phantom Fungus.

Huge Regenerating Phantom Fungus

Huge Plant
Hit Dice: 8d8+64 (100 hp)
Initiative: -2
Speed: 20 ft. (4 squares)
Armor Class: 15 (+9 natural), touch 6, flat-footed 15
Base Attack/Grapple: +6/+24
Attack: Bite +16 melee (3d6+10)
Full Attack: Bite +16 melee (3d6+10)
Space/Reach: 15 ft./10 ft.
Special Attacks: -
Special Qualities: Low-light vision, plant traits, greater invisibility, regeneration 5
Saves: Fort +14, Ref +0, Will +2
Abilities: Str 30, Dex 6, Con 26, Int 2, Wis 11, Cha 9
Skills: Listen +6, Move Silently +9, Spot +6
Feats: Alertness, Improved Natural Attack(bite), Skill Focus(move silently)
Environment: Underground
Organization: Solitary
Challenge Rating: 8
Treasure: None
Alignment: Always neutral