When Albus Potter was about to depart for his first year at Hogwarts he was seen carrying a ferret (or perhaps a Jarvey) in a cage. Neither of those animals are on the allowed list of animals on Hogwarts, so I wonder why Albus was allowed to bring one to Hogwarts. Has the list been updated or discontinued? Or can he do whatever he want because he's the son of Harry Potter and there's a family history (I'm looking at you, Ron) of people not abiding these rules? I've seen a suggestion that this is because Albus is his generation's snooty rich kid whose dad is a bigwig in the wizarding world, but I'm not certain that Harry has anything to do with it.
Here's one consensus from many in the Harry Potter fandom on why Albus Severus, in the movie Harry Potter and the Deathly Hallows - Part 2, is seen with a ferret familiar:
Many believe it's an insider's nod to Harry's rivalry with Draco Malfoy, and a reference to when Mad-Eye Moody turned Draco into a ferret after Draco (in the movie) gets ready to curse Harry when Harry's back is turned. In the book Goblet of Fire, Ron, in particular, is thrilled by Moody's stunt.
‘Don’t talk to me,’ Ron said quietly to Harry and Hermione, as they sat down at the Gryffindor table a few minutes later, surrounded by excited talk on all sides about what had just happened.
‘Why not?’ said Hermione in surprise.
‘Because I want to fix that in my memory for ever,’ said Ron, his eyes closed and an uplifted expression on his face. ‘Draco Malfoy, the amazing bouncing ferret ...’
Goblet of Fire - pages 182 - 183 - chapter thirteen, Mad-Eye Moody - Bloomsbury
I know it sounds from that passage that Ron is the one with the intense rivalry with Malfoy, but I think he's just enjoying the moment -- after all, Ron's been at the receiving end of Draco's bullying numerous times (Weasley is Our King, anyone?). I think it's likely Draco just didn't consider Ron a worthy rival in the same way he did Harry. The seven books demonstrate Harry and Draco's ongoing dislike, although I interpreted canon text as Harry caring much less about what Draco thought of him than vice versa.
Anyhow, right before we see Albus Severus's ferret in its cage, the camera pans over the now grown Draco Malfoy and his wife, who are seeing Scorpius Malfoy off to Hogwarts for Scorpius's first year.
I think it's entirely plausible that the ferret is an inside joke on the part of the movie makers that pokes a little fun at Draco. The ferret does not appear in the books.