I actually think Oda's implying something a little different with the "Beast" and "Not Human" lines.
We saw back in Chapter 1085, that Imu seems to have some sort of "Dragon-like" form. Sort of like the other "Yokai" forms that the Gorosei have.
And I think it's pretty likely that Imu has one of the God Fruits. It's debatable which one it is. Personally, I think the Forest God Fruit is the most likely. Imu lives on Mariejois, which is on top of the Sunlight Tree Eve. Imu spends a lot of time in the Flower Garden. In the Chapter 1138 Mural, we learn that the Forest God was the one who "sent forth Devils", which sounds a lot like Imu's Abyss and Domi Reversi powers. But, that's not really the important part.
The important part is, that the God Fruits are likely all technically different "Human" Fruits. We know the Nika Fruit at least is a "Human" type Fruit. I think it's pretty likely that the other 3 God Fruits are likely also Human Fruits. And we've seen with Chopper, ANYTHING can eat a Human Fruit.
So, here's a idea. What if Imu isn't a human who ate a Devil Fruit that allows them to change into a dragon?
What if Imu is actually a dragon who ate a God Fruit that gives them a human form?
Imu could be a dragon with a human zoan hybrid form.
We know that Dragons existed at one time in this world. Ryuma's one shot Monsters from the Wanted anthology is technically canon. So, real dragons did exist at one point on this world. Imu may be the last dragon left on this world. Or maybe the original "mixed race" result of a Human and Dragon having children. Something along those lines.
Which would mean, what Imu is doing when they Domi Reversi someone isn't turning them into a Devil. It's awakening the Dragon blood that's sleeping inside them. All people with Giant heritage may originally have been the result of mixed breeding between Humans and Dragons. What the Reversi is doing is "Calling them back home" to their original heritage. Tails, wings, and horns are just as common characteristics for Dragons as they are for Devils, after all. And Dragons do have some mythological connections to Devils in the first place. So, there is a bit of overlap, thematically.
Even this latest chapter. Rocks smacks Kaido in his Dragon form down. A "copy" of a Dragon via a Devil Fruit is no match for a "true" Dragon reawakened via Domi Reversi.
Even some of Imu's actions kind of make sense for a dragon. Imu likes to spend most of their day alone in a "cave". They sit atop a pile of stolen treasure, like the Giant Strawhat. Imu seems to have a thing for princesses, like Lili, and dragons do enjoy a good princess kidnapping every once in a while. It's broad strokes stuff, but it's there if you squint.
So, Imu would be called a "beast", because Imu actually isn't human. And dragons were likely treated like monsters and second-class citizens, back when more of them were around. Hunted to the point of extinction.
Now, this wouldn't explain everything about Imu. Because the Abyss powers do seem pretty Devil-coded. But, that could be because they're connected more to a Demon King like Tot Musica from Film Red, that could exist. Or maybe they're just completely unrelated. Because, we have seen other examples of "Symbol Magic" like the Abyss Marks in other places in One Piece. The all-seeing eye charms of the Marys on Wano. Miss Goldenweek's Color Trap paint powers. So, the Abyss powers could just be another power system we really haven't explored yet. Or perhaps, Imu really is taping into the power of a real "Devil". It's just that the real "Devil" of One Piece is a mass of sentient "dream energy" like Tot Musica the Demon King from Film Red.
And the people who created that Demon King may be the Dragons imprisoned within the frozen core of the planet. The Dragons, yearning for revenge, may have given birth to a Demon King that wishes to kill everyone as revenge for their people being hunted by the Humans.
Imu would then be channeling the Demon King's power in order to keep them asleep. Draining off parts of it's power in order to keep it weak and keeping it from fully reawakening and destroying the world. Using it's power, so it can't fully recover it's strength.
More on all that over here, if you want to read more:
https://worstgen.alwaysdata.net/for...message-will-be-the-dragons-are-coming.55673/
As for Imu calling themselves "Mu"...I actually think that's a bit of self-hatred on Imu's part. Imu calls themselves Mu, because they see themselves as "Nobody". Literally. They hide themselves away from the World, they don't really interact with anyone, and they never go outside. Imu's basically a shut-in. Because if I'm right, it seems like Imu's likely trying to stop everyone from reawakening the other Dragons who may be imprisoned at the frozen core of the planet. Imu hates the Dragon inside themselves. Imu may be ashamed that they're a Dragon, and is trying to stop any other Dragons from reawakening and taking revenge on the humans who imprisoned them. Again, look to my old megatheory thread if you want to read more about that.
"The Promise" that Rocks keeps mentioning is likely the promise that Joyboy made to eventually free the Dragons from their unjust imprisonment. Imu and others are against this because they're likely convinced that the Dragons will immediately kill all the Humans in revenge. Which leads to "The Choice" the Strawhats will have to make once they reach Laugh Tale. Would you free someone who was unjustly imprisoned, if you KNEW they were most likely going to kill you afterward? Because years in "prison" changes people.
I could be very wrong about all this, though. Time will tell. I'll admit this is all a little much.