Warning: major Fantastic Beasts spoilers
What do Credence Barebone and Ariana Dumbledore have in common? They were both afraid of their magic and tried to suppress it, and both failed. While their circumstances are different – one is the son of a hateful anti-witchcraft leader while the other had a traumatic experience as a child that left her unwilling to use magic – their reactions are the same. They bottle it up. They pretend it doesn’t exist. And it explodes. In Deathly Hallows, chapter 28, Aberforth Dumbledore describes his sister to Harry:
She was never right again. She wouldn’t use magic, but she couldn’t get rid of it; it turned inward and drove her mad, it exploded out of her when she couldn’t control it, and at times she was strange and dangerous. But mostly she was sweet and scared and harmless.
Yes, I’m saying I think Ariana Dumbledore was an obscurial. It seems logical. But the other thing that connects the two is Grindelwald, who came to New York looking for an obscurus, fully aware of their volatile powers. Where could he have learned about them? Considering how much time he spent with the Dumbledores, Ariana seems like the obvious conclusion. He would have heard Albus Dumbledore’s stories about the strange things that surrounded her, perhaps even known that she inadvertently killed their mother in an uncontrolled burst of power. Furthermore, Aberforth tells Harry that the family kept her hidden and pretended she was a squib because:
If the Ministry had known what Ariana had become, she’d have been locked up in St. Mungo’s for good. They’d have seen her as a serious threat to the International Statute of Secrecy, unbalanced like she was, with magic exploding out of her at moments when she couldn’t keep it in any longer.
This is exactly what Grindelwald tries to do in Fantastic Beasts. That level of destruction and uncontrolled magic is bound to be noticed, and by setting it loose on a big muggle city like New York, he almost guaranteed exposure. Where would he have gotten that idea? The MACUSA government seems to believe that such a thing no longer exists, and even Newt Scamander has only encountered one once before, but Grindelwald had known Ariana and heard even more about her via her brother. His search for an obscurus could very well have begun with her.