The monkey titan has always been the strangest and strongest of all the titans introduced in Attack on Titan (Shingeki no Kyojin). Beyond its beastly look and cold-blooded persona, no one really knows his true identity other than his name (Zeke) and he's highly regarded as the war chief of the Warriors (revealed in Chapter 77).
But no secret will stay forever. As a matter of fact, there are already theories claiming they already identified who really is the monkey titan and the reason why he's so vindictive against the citizens of the walls.
Again there are dozens of fan theories out there waiting to mess up your mind, but I decided to trim it down to what I believe the three most realistic speculations on MT's identity.
Monkey Titan is no other than Grisha Yeager - Yes, we already know that he just turned himself into Eren's chow in order for his son to gain his titan-shifting ability along with the much-coveted power of the coordinate (Chapter 71). But hey, there's a chance that he could be alive, and now in his rebellious state of mind.
Remember, to acquire the coordinate, one need not to devour the wielder to gain the power because it is retained in the spinal cord (Chapter 74). Perhaps, Yeager escaped death and used regeneration ability to fully recover. On the other hand, he might be, well, dead. There's no way Grisha would try to kill the people he took care of for many year, despite the fact that he originally came somewhere outside the wall.
It's Grisha's brother - the striking similarities between Grisha and Zeke are apparent. Both have husky face, great intelligence and not so great eyesight as evidenced by their eye-glasses. Like Grisha's and Eren's titan form, Zeke's Monkey Titan has also that elongated ears. I don't think it's just a mere coincidence Hajime Isayama drew MT with that same set of ears. There has to be a deep reason, and one of it is they are all related by blood.
Monkey Titan is Erwin's father - This is arguably the most feasible of all theories I encountered as far as the identity of the Monkey Titan is concerned. A youtuber in the name of Th3 Black Pho3nix uploaded a video showing evidence that Zeke is the Survey Corps commander's father, who is apparently looking to exact revenge on the monarchy of the walls. Check it out below.
The world of Shingeki no Kyojin is all about mystery and more unsolved puzzles. The manga series is already heading to its 79th chapter, yet not even one got an idea where these titans originated and what the world was before these carnivorous giants went on a rampage. But thanks to the efforts and some wild imagination of these hardcore SNK fans, we may have a little clue on who is the story's 'bad guy' all along.