Gas Spore

A bulbous body floats towards you. It has a central, unblinking eye, and a large maw filled with daggerlike teeth. Smaller eyes, attached to wriggling stalks, sprout from the top of the orb-like body.

Source: Lords of Madness: The Book of Aberrations 3.5
Original Source was AD&D Monster Manual

     The Gas Spore is a giant flying fungal ball. They were created by Beholder Mages. Since their creation they have spread far and wide. Repopulating through their infestation ability. The Gas Spore looks like Beholders and can easily deceive an adventuring party by appearing as so. They are hollow creatures filled with spores under intense pressure to burst.
     The Gas Spore main purpose is to be destroyed. The destruction of the Gas Spore allows it's Death Throe and Infestation abilities to take full effect. Having multiple Gas Spore in an encounter can wreck havoc on a party. On a positive note the spores from this creature can be harvested with a DC 20 Alchemy check. You can turn the Gas Spores into powder and it can be used as a contact poison. Harvesting a living Gas Spore can be tricky and failure could expose you to an infestation.

Combat: The Gas Spore is a CR 3 Large Plant. It's primary abilities are it's death throe and infestation abilities. For a CR 3 creature they have very low Armor Class. Perfect for blowing them up.

-Death Throes: A Gas Spore when it is reduced to 0 hit points explodes. The explosion causes 6d6 points of Sonic Damage in a 10ft Radius. A DC 10 Reflex save is allowed to halve the damage. Characters caught in the explosion are exposed to the infestation ability.

-Infestation: Any living non-plant creature exposed to the Gas Spore's slam or death throw ability must make a DC 12 Fort Save or be infested by spores. The spores rapidly grow and eat flesh. Initial effect is 1d4 Con damage and requires an additional save each hour or lose an additional 1d4 Con. If a character dies from the spores he turns into a formless mush that spawns 1d4 small Gas Spores.
Any ability that neutralizes or delays poison can cure/delay the infestation.

-All-Around Vision: A Gas Spore can see in all directions and cannot be flanked.

-Beholder Camouflage: They get +20 to disguise check to appear as Beholders.

-Flight: Naturally buoyant and can fly 20ft per round.