Sprite

Appearing to be a flying ball of light, Sprites were harmless creatures that could be seen floating through the Black Mesa Research Facility. They eventually all led to the portal that brought the Gene Worm in, evidently acting as energy conduits to construct and charge the portal to the world of the Race X aliens. It is unknown if these are biological aliens or technological creations of Race X.

In Half-Life: Opposing Force, sprites acted as guides to show objectives which were otherwise unclear.