When Joyce Byers and Jim Hopper entered the Upside Down to search for Will Byers, Hopper found and briefly studied a broken eggshell near Castle Byers that appeared hatched; it was hollowed out and cracked open at the top.
It is currently unknown what hatches out of these eggs - it doesn't seem to be Demogorgons, as at least one grew from a larvae implanted inside Will, implying that all Demogorgons reproduce in this parasitic manner.
Behind the scenes
Aaron Sims Creative created the concept art for the egg. The company Fractured FX, who made the prosthetics for Will's fake body and Barbara's corpse, created the prosthetic for the egg as it was seen in Chapter Eight.
In the closing moments of Chapter Five, Nancy enters the Upside Down and witnesses the Monster being hunched over something, appearing to feed off it. In an interview, visual effects artist Aaron Sims implies that the Monster was feeding off an egg in this moment. Despite this, close inspection of the scene suggests that it was instead feeding on the wounded deer Nancy and Jonathan found earlier in the episode. 
- ↑ "Chapter Six: The Monster"
- ↑ "Chapter Eight: The Upside Down"
- ↑ 3.0 3.1 "Exclusive ‘Stranger Things’ Art Reveals ‘Upside Down’ Secrets and Barb’s Alternate End" ScreenCrush. August 22, 2016.
- ↑ "The Egg we created for STRANGER THINGS!" Facebook: Fractured FX. August 19, 2016.
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