This could be completely off, but I can’t stop thinking about what happened to Eloise. We see her run out of the root cellar to chase Miranda, and then she’s never seen again. Victor later tells Henry that he thinks Eloise’s body is buried under the rock next to Miranda, but even he doesn’t sound certain. He admits Eloise was so small that her remains were basically destroyed.
Earlier in the series, when Tabitha and Victor are in the tunnels, something small and creepy lunges at her from behind a set of bars. It’s clearly angry and not quite like the other monsters or the Anghkooey children we’ve seen. Then later, when Victor and Henry return to the tunnels to retrieve Jasper, there’s that strange tea party setup: a small table with teacups, Jasper seated at the head like a doll, and random items scattered around. Henry even finds a piece of Miranda’s clothing and questions why it’s there, but Victor brushes it off, saying the monsters “like to take things.”
There’s also the moment when the monster woman warns Victor to be careful going into the tunnels because one day they might make him stay. She doesn't say they'll kill him like they kill everyone else, she says deliberately that they'll make him stay.
What if the monsters found Eloise the day of the massacre and, instead of killing her, took her into the tunnels? Maybe the small creature behind the bars is Eloise. It could explain the childlike setup with the tea party, Jasper positioned like a tea party doll, and the collection of random objects, including something that belonged to Miranda.
Maybe the monsters keep her as some kind of twisted “pet,” or maybe it ties into their past whether that’s guilt or loss or whatever. I also noted that we only saw the thing behind the bars when Tabitha was in the tunnels. We didn't see it at all any other time and it would make sense if it was Eloise then she could be angry with Tabitha AKA Miranda.