An egg sac was a case which contained eggs.
An adult Xindi-Insectoid could produce multiple egg sacs asexually. (ENT: "Hatchery")
The Crepusculans also produced egg sacks. One cluster off egg sacks was placed outdoors in a desert canyon, supported on stick-like legs and connected by lines. (DIS: "The Vulcan Hello")
These egg sacks appeared to be notably larger then a Crepusculan seen moving on top of them.