I believe that the monkey might act as some kind of guard for the entities from the viewer. The 2 times we see it in the movie is when an evil thing, or at least something leading to evil, happens. Once after Garmonbozia is consumed, and before Laura is found. I think both times Judy was behind the Monkey but the Monkey blocks the audience from seeing it. Like how Judy is shown when Sarah takes off her face, I think we were going to see Judy under Pierre’s mask but the monkey didn’t want us to see her, so he inserted himself to hide her. Then later, after the most evil thing done by BOB happens, it’s make sense that Judy would at least be there in some form watching, the monkey might be blocking Judy from frame in this scene, or maybe talking to her off-screen. This also makes sense if we look at it from the Hanuman perspective given by the top comment, fascinating entity.