Langolier

The Langoliers are medium-sized globular creatures consisting almost entirely of a huge sharp-toothed maw. They have been described as "time keepers" and inhabit a temporal realm usually unreachable by human senses, to which non-living matter drifts into as the linear timeline advances from present to future.

This is fundamentally different from how most people imagine the past: in the Langoliers' world, when we move ahead in time as future becomes present and present becomes past, we leave behind a "past realm" which looks and feels like an uninhabited and used-up copy of the present world. The voracious Langoliers thrive there in huge swarms, consuming all matter and allowing it to be recycled into the "future world" which is being created temporally ahead.

Occasionally, a temporal anomaly might transport unfortunate citizens to the past and into the realm of the Langoliers, which will rapidly attempt to consume the intruders along with the entire "past world" around them. However, while it is possible for people to pass through the anomaly unharmed (as long as they are asleep, although the reason for this is unknown) the same is not true for the Langoliers, who dare not trespass into the present time themselves. This is a relief, because with their insatiable hunger for reality-consumption, chaos would certainly ensue if they did.