Egg Sacs are a trap found in Sunken City, presumably laid by Giant Flies. They are the Spelunky 2 version of Worm Eggs from The Worm.
When a player or other entity, such as a Frog gets close enough to the Egg Sac, it will start to wriggle before popping, revealing three Grubs.
Running into the Egg Sac will stun the player, while being too close to the Egg Sac as it ruptures will cause the player to drop their held item. The Egg Sac will reflect any bombs thrown at it.