before you ask no it isn't pit bonnie at all.

well in a video(link in the title) Unwithered Truth tells a lot (ALOT) about this charecter and let me tell you,it's very complex.

the Rabbit is a creature that is created by the truamas and bad actions that William did in this series and over the time those agonies and stuff like agony that he generates come toghether to create this fictitious creature named The Yellow Rabbit

the Yellow Rabbit and William are kinda like Bruce Wane and BatMan they are the same charecter but they have diffrent personalities William and The Yellow Rabbit are like that (they are the same charecter with diffrent personalities so when the text changes from purple to yellow it means that the yellow rabbit(William's evil personalty) is speaking

but Unwithered Truth said that this doesn't mean like we will have a William like the movies (like how Yellow Rabbit(fnaf movie) and William(fnaf movie)were two compelety diffrent charecters (William was smart and the Yellow Rabbit was brutal)) so we don't have to be worried about that

there's so many reasons why The Yellow Rabbit well..looks and called Rabbit one of them is because William wore a costume that was a rabbit when he did the MCI another is that when William was a child he had a rabbit and he somehow(we don't know how) died (likely by William)and then William(& maybe his father) dug it a grave in their garden and that action of William(&maybe his father)became a trauma of William and that trauma could have helped William to make this creature in his mind and memories

there's even a human face in The Yellow Rabbit's mouth and that can be William's face

note1:The Yellow Rabbit first got reffrenced in an ARG video of fnaf:anniversary

note2:this post is more a summary of what i understood from what Unwithered said than an actual theory even tho I said it's a theory

Note3:if you didn't understood a word of this post i recommend you to watch the video (24:25 to the end are the parts that they are talking about this the most and i used this part the most)