I propose an idea that may seem ridiculous, but is actually quite plausible in hindsight. That being the idea that Mo is the reincarnation of Sha Wujing alongside Sandy.
While this particular theory doesnāt have a large amount of clues to back it up, I believe thereās still enough to paint a clear picture.
Moās appearance; it goes without saying that Mo is an unusual cat based off his fur alone. What makes it more notable is the fact that he perfectly matches Sandy. Thanks to the season four sets which included several kitten Mo figures we know for a fact that Moās fur color (mohawk included) is his natural fur color.
Intelligence and personality; once again it goes without saying that Mo is an unusual cat with his intelligence being of large note. Heās able to fully understand the people around him and react accordingly. Not to mention heās able to handle many forms of machinery with ease. Additionally Mo and Sandy are extremely similar in personality, with the only differences being that Mo is more cautious and is quicker to bouts of aggression than Sandy is.
During Wukongās bout of amnesia he mistook Mo for Wujing, now many would assume that itās because of his appearance or the interruption of the transcendental meditation having scrambled his brain slightly as else see with him mistaking a stick for his staff, but I believe itās actually because Wukong (even in his scrambled state) could recognize Wujingās essence inside Mo and reacted to him accordingly.
We saw during the events of season 4 that when Pigsy tried to awaken Sandy similarly to how he and Tang were awoken, it failed completely. It wasnāt until Mo fell into his arms that he was awoken. I believe this was because Sandyās soul couldnāt be awoken during that time because not all of his soul was present, until Mo appeared and it was finally able to work.
Sandy is able to completely understand Mo as if heās speaking the same language. Sandy is never shown to be able to do this with other creatures or other cats. Only Mo.
Last but certainly not least timeline; we donāt know when Sandy got Mo, but it is inferred that Mo is Sandyās first therapy cat and as such we and infer that Sandy has had him for years. Which given the fact that Mo is still very spry and energetic wouldnāt make sense. Unless the two were connected in some way which causes Mo to age differently than other cats.
Now as to why or how Wujingās soul split is a complete mystery. But I can say with confidence than Sandy and Mo have always been connected in a deeper manner than any other characters. And hopefully soon weāll be able to get more clues to this connection.