I did not write this theory, I just want to say that I'm happy that other people as well find similarities between the person that is the Ape Titan and Eren's father. Enjoy 
"I truly believe the Beast Titan is related to Eren. There seems to be a recurring pattern with bloodlines and clans in this series (Ackerman, Reiss, the Asian clan, Ymir’s people, the Shifter Village) and I wouldn’t be surprised if the Yeager family also plays an important role in the story.
In my opinion, the Beast Titan’s human form looks similar to Grisha:
And there’s even a direct focus on his glasses, which also look similar to Grisha’s:
After some thought, failed attempts at math and figuring out timelines, this is the theory I choose to believe for the time being: the Beast Titan is Eren’s great-grandfather.
You might think he’s not old enough, but I have a theory that shifters don’t age so long as they’re in titan form (and we already know titan’s can’t die of hunger or natural causes, so this might be how the Beast Titan survived outside the walls for so long). When Ymir was trapped in her mindless titan, she wandered outside the walls for 60 years before finally gaining her shifting abilities, and she didn’t age (or perhaps she aged so incredibly slow she only transitioned from a child to a teenager within the time span of 60 years.) So, it’s possible that the Beast Titan just looks the same age he did when he first transformed, but in actuality he’s over a hundred years old.
We all know there’s something fishy going on with the Yeager family because Grisha had personal access to titan serums. The Beast Titan could’ve been Grisha’s grandfather and
A) injected himself and became a mindless titan, eventually ate another shifter (perhaps from the Shifter Village and that’s how he knows Reiner and Bert) and regained his human consciousness or
B) was the very first titan and didn’t actually need to eat anyone to transform at will (I’ll elaborate more on this in a bit).
If it weren’t for the Beast Titan’s oblivion toward modern equipment, I would’ve assumed he was closer related, like maybe Grisha’s brother, but him being Eren’s great-grandfather would not only make more sense, but actually be more beneficial to the plot because he would know SO MUCH about the history of titans and humans because he would’ve actually lived through it.
Due to his long absence from humans, it makes sense that he would lack knowledge about 3DMG. He must’ve left the walls before the years 775-788 (according to Before the Fall, the 3DMG was made within this time frame).
In the start of the series the year is 850, which would mean he was outside the wall for no less than 75+ years. Considering he had to raise an offspring (Grisha’s father?) I’m guessing he was maybe 40/50 years old by the time he left the wall, indicating he’s 120+ years old.
According this this, the titan’s appeared 107 years ago. If the Beast Titan is as old as I think he is, this indicates that he was present when the titans and walls first appeared–or the Yeager family was directly related to, or caused the creation of the titans (oh, the irony). Levi’s great-grandfather mentioned he was born after the walls were already constructed, which was why he didn’t know everything, but I think the Beast Titan witnessed/caused the near-extinction of the human race, was present when the walls formed, and later inexplicably left beyond the walls. In other words, this guy knows everything.
It’s speculated that the Beast Titan turned Connie’s village into titans, and so far this is the only method (other than injecting serums) that could change humans into titans. The Beast Titan has this incredible power, and he’s viewed as the alpha; other titans are submissive and fearful toward him:
Mindless titans don’t try to eat him, they take orders from him, and he also doesn’t need sunlight to be active. I am almost 100% certain that the Beast Titan is the very first titan.
I also want to point something else out that connects the Beast Titan to the Yeager family. Eren, Grisha, and the Beast Titan all have pointy ears:
I don’t think this is just a coincidence. Even Eren’s mindless titan had this feature:
This feature seems to be exclusive to those who use the “Yeager serums”. None of the other titan shifters (or even mindless/abnormal titans) have this feature–except for Ymir.
If my calculations are even remotely close to correct, she would’ve been present somewhere around the Beast Titan’s generation. Now, I think assuming Ymir is related to Eren because of this detail is a stretch, but another possible connection is Ymir was experimented on with the Yeager serum, or she just (for some unknown reason) stole the serum and injected herself with the Yeager-exclusive serum, hence the pointy ears.
I think there are different type of serums, perhaps exclusive to each family (like the Reiss serum, Yeager serum, Bert/Annie/Reiner serum). Unlike Annie, Bertholdt and Reiner, who are all skinless, Ymir, Eren, Grisha and the Beast Titan all have skin–the Beast Titan is covered in hair, but I think this is due to the fact that he is the “original titan”. According to evolution, humans evolved from apes. It would be fitting if the shifters are descendants of an ape-class.
Grisha definitely plays a huge role in this story–the biggest mysteries in this series are
A) the contents of the basement
B) how Grisha had access to titan serums
C) Grisha’s role and intentions.
Despite him no longer being alive, Grisha still holds the key to the mysteries and Eren knows this, and this is why he’s going to speak with Keith: the only man who might know about his family history.
Keith obviously knew Grisha personally:
This is a very personal line that indicates he had close ties with Grisha and might know a lot about the Yeager family. Grisha might’ve entrusted him with information about the Yeager’s role in all this, and perhaps he told him about his connection with the Beast Titan. I have a feeling Keith may shed some light on us, and when Eren and Co. finally get to Shiganshina, the Beast Titan will reveal much more. And perhaps their connection, if they have one, will be addressed as well."
"I truly believe the Beast Titan is related to Eren. There seems to be a recurring pattern with bloodlines and clans in this series (Ackerman, Reiss, the Asian clan, Ymir’s people, the Shifter Village) and I wouldn’t be surprised if the Yeager family also plays an important role in the story.
In my opinion, the Beast Titan’s human form looks similar to Grisha:
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You must be registered for see images
You might think he’s not old enough, but I have a theory that shifters don’t age so long as they’re in titan form (and we already know titan’s can’t die of hunger or natural causes, so this might be how the Beast Titan survived outside the walls for so long). When Ymir was trapped in her mindless titan, she wandered outside the walls for 60 years before finally gaining her shifting abilities, and she didn’t age (or perhaps she aged so incredibly slow she only transitioned from a child to a teenager within the time span of 60 years.) So, it’s possible that the Beast Titan just looks the same age he did when he first transformed, but in actuality he’s over a hundred years old.
We all know there’s something fishy going on with the Yeager family because Grisha had personal access to titan serums. The Beast Titan could’ve been Grisha’s grandfather and
A) injected himself and became a mindless titan, eventually ate another shifter (perhaps from the Shifter Village and that’s how he knows Reiner and Bert) and regained his human consciousness or
B) was the very first titan and didn’t actually need to eat anyone to transform at will (I’ll elaborate more on this in a bit).
If it weren’t for the Beast Titan’s oblivion toward modern equipment, I would’ve assumed he was closer related, like maybe Grisha’s brother, but him being Eren’s great-grandfather would not only make more sense, but actually be more beneficial to the plot because he would know SO MUCH about the history of titans and humans because he would’ve actually lived through it.
Due to his long absence from humans, it makes sense that he would lack knowledge about 3DMG. He must’ve left the walls before the years 775-788 (according to Before the Fall, the 3DMG was made within this time frame).
You must be registered for see images
You must be registered for see images
It’s speculated that the Beast Titan turned Connie’s village into titans, and so far this is the only method (other than injecting serums) that could change humans into titans. The Beast Titan has this incredible power, and he’s viewed as the alpha; other titans are submissive and fearful toward him:
You must be registered for see images
I also want to point something else out that connects the Beast Titan to the Yeager family. Eren, Grisha, and the Beast Titan all have pointy ears:
You must be registered for see images
You must be registered for see images
You must be registered for see images
I think there are different type of serums, perhaps exclusive to each family (like the Reiss serum, Yeager serum, Bert/Annie/Reiner serum). Unlike Annie, Bertholdt and Reiner, who are all skinless, Ymir, Eren, Grisha and the Beast Titan all have skin–the Beast Titan is covered in hair, but I think this is due to the fact that he is the “original titan”. According to evolution, humans evolved from apes. It would be fitting if the shifters are descendants of an ape-class.
Grisha definitely plays a huge role in this story–the biggest mysteries in this series are
A) the contents of the basement
B) how Grisha had access to titan serums
C) Grisha’s role and intentions.
Despite him no longer being alive, Grisha still holds the key to the mysteries and Eren knows this, and this is why he’s going to speak with Keith: the only man who might know about his family history.
Keith obviously knew Grisha personally:
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