Fantastic Beasts: The Crimes of Grindelwald may explain the backstory to Voldemort's snake, Nagini. The prequel film series is ostensibly focused on Grindelwald's rise to power, and the terror he inflicted upon the wizarding communities across Europe during the 1930s and 40s. However, it seems as though J.K. Rowling is also working on weaving in many elements from Harry Potter into the Fantastic Beasts story, so that eventually the whole arc of 13 movies, from Fantastic Beasts and Where to Find Them, to Harry Potter and the Deathly Hallows Part II, will exist as a complete whole.
We know Grindelwald's reign of terror came to an end in the infamous duel between him and his former lover, Albus Dumbledore, in 1945, when Dumbledore emerged triumphant and took ownership of the Elder Wand. From there, Voldemort eventually rose to power as the darkest wizard of all, until a baby boy named Harry Potter almost destroyed him in 1981. Rowling has always been a master at laying strong foundations and backstories to all of her work, and that's becoming ever more evident in Fantastic Beasts. In the lastest trailer for The Crimes of Grindelwald, we meet Nicolas Flamel, immortal alchemist and creator of the Sorcerer's Stone, which was the first artifact that Voldemort sought to help him regain strength. But there's another connection to Voldemort lurking in Fantastic Beasts: The Crimes of Grindelwald, and it's something (or someone) that goes on to hold great significance throughout all of Harry Potter.
The unknown character played by Claudia Kim has no name billed other than Maledictus; in the Wizarding World, someone who will at some point turn into a beast. She resides in the circus that Credence joins, and all things point to her eventually turning into a snake named Nagini.
Nagini was a large, female snake who was always by Voldemort's side. In part, this was because she was one of his Horcruxes, and so, as Dumbledore suspected, Voldemort kept her close by in order to preserve the life fragment within. She was eventually killed by Neville Longbottom during the Battle of Hogwarts.
Nagini's origins, though, have always been unclear. It's not even known if Voldemort owned the snake before his downfall at the hands of Harry Potter in 1981, or if he came by her when he was hiding in the forests of Albania. However, what we do know is that Voldemort used Nagini's venom (obtained by Peter Pettigrew) to sustain him in a temporary form until he could regain his body. She was turned into a Horcrux after Voldemort murdered Bertha Jorkins in 1994.
The lack of backstory for Nagini is curious; Rowling is meticulous in making sure all major characters and creatures in her stories have them, and for Nagini's to be so vague would point towards Rowling planning on telling it at some point. Certainly, Nagini will have a complete backstory, even if it's only Rowling who knows about it for now. Could that all change with Fantastic Beasts: The Crimes of Grindelwald?