If it spawns in the Overgrowth and Dilapidated Halls. Then you're looking for the Bloboomi. https://charcuteriedev.com/Pages/Ghosts/Gallery/Bloboomi
You're gonna have an easier time trying to get to spawn in Overgrowth, just gotta be lucky. And if you don't just go back to the menu of the game and try again.
Please don't try to get to spawn in Dilapidated Halls. It has a 1 and 4096 chance to spawn there, I only got so lucky to get it spawn on random luck alongside that scary monster that jumpscares you randomly, which gives you one of the two secret achievements
OH! And before I forget, this enemy can't damage you but can still kill you if you stay inside it for too long. If you have the Endosomatic Heart, you can feed it and add it to the Fed part of the Ghostiary. Make sure you do both that and eat it when you encounter it if you were to encounter two of them! It may have a chance in appearing in Overgrowth during a Blackout as well.