The answer is NAD+.
In the process of glycolysis, NAD+ is reduced to form NADH+ and H+. If NAD+ is not present, glycolysis will not be able to continue. Under anaerobic conditions, the regeneration of NAD is coupled to the reduction of pyruvate. The anaerobic regeneration of NAD is called fermentation.
In animal tissues , fermentation occurs as pyruvate acts as a makeshift hydrogen acceptor and gets reduced to lactate when NADH transfers electrons to pyruvate. This enzyme catalyzed reaction occurs whenever the available oxygen has been consumed.