Would it be possible, through genetic engineering or on some alien world, to have a creature with a biological rebreather? That is, a method of recycling the oxygen in a closed circuit, so as to maximize its usage and the time you can "hold your breath". Another example would be oxygen recycling in space stations.
This was intended for a creature with limited access to oxygen, but access materials used in rebreathers to recycle oxygen.
Presumably, it would need to have a special diet to replace the oxygen-recycling materials. Or, perhaps the creature would have some kind of microbial plant life inside of it, that turns CO2 back into oxygen?