From Creatures Wikia
ATP (Adenosine triphosphate) is a vital chemical in Creatures 3 and Docking Station, used for almost every aspect of metabolism in a creature. It degrades into ADP when used, which can be converted back into ATP by a creature's natural breaking down of glucose. Without ATP, the creature would drop dead near-instantly. A poison called ATP Decoupler takes advantage of this, which, even in the smallest of its doses, can kill a creature within seconds.
It is also a real chemical.