Ability: Leaf Absorb
Restores one quarter of max HP to the user when hit by a grass-type attack.
Pokemon: Exeggutor, Bellossom, Tropius, Roserade, Cherubi, Cherrim, Carnivine
Ability: Earth Armor
Sharply raises user's defense in a sandstorm.
Pokemon: Sandshrew, Sandslash, Onix, Steelix, Gligar, Gliscor, Shuckle, Mawile, Baltoy, Claydol, Lunatone, Solrock, Gible, Gabite, Garchomp
Ability: Sludge Boost
Sharply raises user's attack in toxic fumes (see below).
Pokemon: Oddish, Gloom, Vileplume, Tentacool, Tentacruel, Grimer, Muk, Qwilfish, Gulpin, Swalot, Seviper, Stunky, Skuntank, Skorupi, Drapion, Croagunk, Toxicroak
Weather Condition: Toxic Fumes
Is activated upon use of the move Nuclear Gas, which for all intents and purposes is a poison-type version of Rain Dance/Sunny Day/Hail. For five turns, potentially lethal gas seeps from cracks in the ground to fill the battlefield. Every turn there is a 20% chance that a Pokemon will become poisoned, though poison- and steel-types are immune to this effect.
In addition, poison moves do 50% more damage, and ground moves do 50% less damage. Castform also becomes a poison-type (essentially a pinkish pile of sludge-like material).
...whew, that was some work.