Creature design for a contest I won this December.
The creature will be part of the bestiary of an RPG game called Xenoverse, published by TinHatGames.

DESCRIPTION:

"Despite its small size, the pneumocephalus possesses unique properties. With an average weight of 25 grams and by means of an inflatable bag placed behind the head, this tiny creature is able to float in the air. The secret of this property lies in its diet and in a rapid metabolism resulting from its small size.
The pneumocephalus has in fact two different intestines: one used for normal digestion and one specialized in the management of particular toxic microalgae that grow in abundance in their natural habitat. Instead of being digested, this algae are instead "cultivated" inside the intestine, which is able to absorb the sulphur during the photosynthesis and therefore change their metabolic activity, so that the algae produces hydrogen instead of oxygen as residual element. The sulphur provides resistance and elasticity to skin and tendons, which is necessary for the bag inflation. Due to its heaviness in relation to air, it is also used under gaseous form to inflate some micro-zavores contained within the bag, with which the pneumocephalus manages stability during the fluctuation.
Hydrogen, being lighter than air, is instead used to inflate the bag, allowing the peumocephalus to fluctuate, or is discarded if not necessary. The trunk and the lateral siphons placed around the head of the pneumocephalus have a dual motor-respiratory function: the air is channeled into the lungs through the trunk and exhaled by the siphons generating a propulsion.

The danger of this animal comes from the toxin produced by the algae of which it is immune, and which is able to use as defense system through a poisonous sting placed at the base of the tail. Although one single specimen is almost harmless, their danger lies in the number of which a swarm is composed. The pneumocephalus has several weak points: first of all, it is extremely “flammable” due to the ammount of hydrogen gas contained in the sac, wich, in contact with energy sources such as fire or electricity, can give rise to explosions and chain reactions depending on its proximity to other specimens. In addition, it is subject to a very short life cycle, as a result of its high metabolism.

If a player is able to stun it during the fluctuation (i. e. before the hydrogen is completely exhaled), the pneumocephalus can be tied to prevent the gas from exhalating. Subsequently, the carcass, still loaded with hydrogen can be used as a sort of organic explosive."