I give you...SWAMP THING!
I threw this together for another DM a little over a year ago, and never got to use it myself, so I can't say how balanced it is. Also, it's pre-3.5. But maybe you (or anyoen else) can get some use out of it.
ELEMENTAL, SWAMP
SIZE/TYPE: Large Elemental
HIT DICE: 16d8+112 (184 hp)
INITIATIVE: -1
SPEED: 30 ft., swim 30 ft.
AC: 20 (-1 size, -1 Dex, +12 natural)
ATTACKS: Slams +21/16/11
DAMAGE: Slam 2d8+15
FACE/REACH: 5 ft. by 5 ft./10 ft.
SPECIAL ATTACKS: Destroy objects, duplicate self, spell-like abilities
SPECIAL QUALITIES: DR 10/+2, elemental qualities, fire vulnerability, regeneration 3, spell-like abilities, travel through the green
SAVES: Fort +17, Ref +4, Will +8
ABILITIES: Str 30, Dex 8, Con 25, Int 12, Wis 16, Cha 17
SKILLS: Hide +15*, Knowledge (Nature) +9, Listen +11, Sense Motive +11, Spot +11, Swim +18, Wilderness Lore +11
FEATS: Blindfight, Cleave, Power Attack, Sunder
CLIMATE/TERRAIN: Swamp
ORGANIZATION: Solitary
CHALLENGE RATING: 12
TREASURE: None
ALIGNMENT: Always Neutral or Neutral Good
ADVANCEMENT RANGE: 17-24 (Huge)
DESCRIPTION
Unlike most elementals, swamp elementals seem native to the Material Planes. Appearing as large, vaguely humanoid creatures composed of mud, vegetation, and other swamp detritus, they often act (knowingly or unknowingly) as the guardians of their boggy homes. Some scholars theorize swamp elementals are created by nature in response to a great need, such as an impending invasion or unnatural catastrophe, while others believe they are created accidentally, an “aberrant” elemental, but neither theory has been conclusively proven.
Despite their fearsome appearance, swamp elementals are largely peaceful and sedentary. If left alone, they will remain rooted to one spot for centuries, communing with nature and higher intelligences on the elemental planes. However, when the swamps are endangered they are roused from their slumber and attack with the elemental fury of a hurricane. Druids revere swamp elementals and often designate their domains as holy places, erecting rune-covered pillars to warn away travelers who might inadvertently draw the elemental’s wrath. In times of war, however, the druids may purposefully lead or drive an enemy into the elemental’s territory.
COMBAT
Swamp elementals possess great strength and can do terrible damage with their knotted, club-like fists. However, their true power lies in their ability to manipulate plant life (including their own mostly vegetable forms) and the environment.
Destroy Object (Ex): A swamp elemental that takes a full attack against an object or structure deals double damage.
Duplicates (Sp): As a standard action, a swamp elemental can animate vegetable matter within 180 ft to form a creature like itself. This effect occurs at 12th caster level. It takes one full round for this duplicate to form, after which it can move at a speed of 20 ft and attack. The duplicate is in all physical ways like the originating swamp elemental, but it lacks the latter's spell-like and supernatural abilities. A swamp elemental can control two such duplicates at a time. If the elemental itself is slain or incapacitated, the duplicate collapses into inert matter. A mortally wounded swamp elemental can pass its consciousness into a duplicate (which essentially becomes the original elemental in every way), but this takes three full rounds of concentration.
Elemental Qualities: Immune to poison, sleep, paralysis, and stunning. Elementals are not subject to critical hits or flanking.
Fire Vulnerability (Ex): Swamp elementals are partially susceptible to fire, taking 1 additional point of damage per die from fire-based attacks.
Regeneration (Ex): A swamp elemental in contact with vegetable matter regenerates damage. Fire and cold do normal damage to it. If a swamp elemental loses a limb or body part, it regrows in 3d6 minutes. It can reattach the severed member by holding it to the stump.
Spell-like Abilities: At will - Detect Animals or Plants (only plants), Speak with Plants; 3/day - Summon Swarm; 1/day - Diminish Plant, Plant Growth. These abilities are used as cast by a 5th lvl Sorceror (DC 13 + spell level). Although it is not a true plant, a swamp elemental may use the Plant Growth spell on itself. The result is to halve or double its HD and increase/decrease its size one increment, with changes to ability scores, hp, attacks, saves, as per the MM. This effect lasts only 1 minute.
Skills: *A swamp elemental receives a +12 racial bonus to hide checks within the bounds of any swampy area (included above).
Travel through the Green (Su): A swamp elemental can enter the vegetation below it as a full-round action and reappear anywhere else that there is vegetation. Within the bounds of its own swamp, this ability is usable at will and has no range limit. Outside its swamp, it is equivalent to a tree stride spell cast by a 9th lvl druid, with a range of 500 ft and usable through any vegetative surface.