RXQ

Origins
RXQFSFASXB(Mistakenly referred to as "Shadow Bonnie") is a secret character very rarely seen in the PC version of FNaF 2 on any night. RWQFSFASXB may look similar to Bonnie, warranting the nickname, however the teeth are actually buck teeth compared to toy Bonnie who has a full set of teeth. Upon Encountering RXQ during FNaF 2, the screen will go dark and the only thing visible would be the glowing teeth of RXQ in the corner of the room. After a few seconds, the game would crash.

The reason for this animatronic being introduced to the game is unknown, and none, if any, story info has been revealed about this character.

UCN
A big reason RXQ might actually be canon and not just a mistake is because RXQ makes ANOTHER appearance in Ultimate Custom Night, when summoned by JJ.

RXQ doesn't actually kill the player in custom night, however it instead makes the screen very dark, meaning that we can't get dialogue from this character.

FNaF World
RXQFSFASXB is also a character in Scott's spin-off RPG, FNaF World. This character is unlockable through normal fighting mechanisms, and has three attacks, all seen in other animatronics: Mimic ball, Haunting, and Gift boxes.

Conclusion
I personally think that whether Scott has to add story to this character later, or the story is undiscovered, one thing is certain: RXQ is just as important as Golden Freddy. They both had the same origins, however Golden Freddy had more story and detail, leading him to receive more attention. Yet through all of this, RXQ is also as powerful as Golden Freddy. Both can crash the game, both can mess with the night itself, and they both appear the same way: they randomly appear.

Either this is just another useless animatronic with no meaning whatsoever, or we have been overlooking this animatronic too much.