Well (and I saw it over the weekend too), this villain take place in the mainline Potter books too.
Now he never appears directly, as such, but is a figure in the background of the movies.
[spoiler]Grindelwald was mostly seen in a few scenes in the Deathly Hollows and Half Blood Prince. If you remember in that, when Voldemort is looking for the Elder Wand, the end game macguffin wand, the one Dumbledore posesses. He is killed when Voldemort comes looking for it, knowing that at one time he had it. In fact, if you pay attention, the necklace that he gives Credence is the Deathly Hollows symbol.
Remember that? The triangle with a circle inside with the wand inside that? The thing Harry spends time looking for to defeat Voldemort? Well Grindelwald is also part of that story. The Elder Wand is one part of that. So that necklace is where my suspicions were confirmed,a s it were.
And it is only vaguely hinted at in the books, and more vaguely in the movies, but that the Deathly Hollows were part of a plan to reveal wizardry to the world to usher in a (benevolent?) wizard ascendancy. He wanted to force wizards public so as to induce political revolution that would make them the ruling elite.
So it makes more sense knowing that he had been a friend, lover, and ally of Dumbledore who had a falling out over this power play. A falling out that killed Dumbledore’s sister. This movie takes place after that, I think. Hard to say for certain. But he was an established radical in the books, and this is merely expanding on what was a minor footnote there.
So his motives, and plans, make a fair amount of sense, if you know a bit about him. They even make sense on a very surface level once you realize his endgame is explicitly about forcing wizardry knowledge public. Why? That requires the background I gave, but what does make sense within the film I felt.[/spoiler]
However this is something I tend to agree with. The Potter universe did tend to mix and match broadly with general mythology and unique to their universe. But the ratio does seem skewered towards more unique to this universe creatures. Now that’s fine, if those serve a purpose. The Obscurus, for example, is perfectly fine being a new creature. But the magic rhino could have easily been something culled from traditional myth.
Given I had just finished American Gods, the Thunderbird was a rather nice touch I thought.