Here are some suggestions for breath weapons for monsters found in the 3.0 MM. These additions to creatures, not changes to monsters that all ready have breath weapons.
Assassin vines could have pollen that causes the victims to fall asleep, induce nausea or even enrage violet fungi that happens to live near the vine (the fungi will not harm the vine even in this state). For a standard vine, I would make the pollen a cone 30' long by 10' wide.
Bugbears blessed by their gods may gain a breath that helps them in the hunt. It is a small cone, only 10' long by 5' wide, and dazes, stuns or causes confusion for d4+1 rounds. The breath changes as the bugbear gains levels.
Chuuls can have a few nasty breath weapons. The most common, ironically, is one that causes dehydration. The victim suffers damage and has an overpowering urge to dive into the water and drink. Chuuls with this usually poison the water to make their kills that much easier. Another causes rust and is most commonly found in chuuls that live in salt water. The most vile breath weapon found among chuuls is one that corrupts. It is extremely powerful, turning the victims into half chuuls (Silverthorne's Template Troves II). Only the oldest and most powerful chuuls have such an ability. All are cones, the first two are 30' long by 10' wide and the last is 60' long by 20' wide.
Driders, phase spiders, other arachnids and even some venomous animals (such as dinosaurs) can release their poison as cones of tiny droplets. Driders can also spit a substance that lures arachnids to those struck. They use it to track enemies as well as ensuring a kill even if they are slain. Drider breath weapons are 20' long by 10' wide.
Gargoyles can have one of two breath weapons. One causes petrification or its reverse (so the prey can be eaten later). The other causes paralysis and forms an illusion over those affected. Victims seem to vanish or turn into trees, statues or something else. Their breath weapons are 30' long lines.
The rare few green hags that have breath weapons gained them through magical and alchemical self experiments. Most have a version that charms victims (Charm Monster), though some transform their victims into lichen covered plants (one has to scrap away the lichens to see the victim's face as well as loot) and a few can even breathe poison like giant spiders. Their breath weapons are random in shape because of the weapons' origins.
Illithids can breathe a form of their slime. Those struck and fail their save are much easier to affect with telepathic attacks (DC +4 or more). The slime is a 25' long line.
Nagas can get very old and some use a breath weapon that forces the victims to watch the naga's entire lives. It is called sight of ages and for every century the naga has lived, the victim stands still for two rounds. Other nagas can summon serpents with their breath weapons, bringing 1 hit die of snake per 5 hit dice of naga. Both are cones 30' long by 10' wide (the naga selects where the serpents appear within the cone).
Otyughs are just gross and their breath weapons reflect that. Stinking Cloud, Insect Swarm and Contagion well represent the effects of a gulguthra's breath weapon (though the swarm is permanent). All of them are cones 40' long by 20' wide.
Troglodytes are an odd species to have a breath weapon, especially since they already have their stench attack. Tinkering by dragons or wizards or just magical pollution may be the origin of these three breath weapons. The first two involve fear, as per the Cause Fear and Fear spells. The trogs use them to drive off opponents or drive them into an ambush. The other causes mindless rage. Everyone affected is confused and rages. It isn't terribly popular as the victims may very well kill the trogs that caused the rage. All three are cones 10' by 5' wide.
What monsters would you give breath weapons?
