One of the released screenshots shows the titular heron with horrifying and uncanny human teeth and gums in its beak, which makes it look almost like an avian version of Nigel Thornberry.
Overall the heron before he turns into a much goofier design, is subtly off in spite at first glance, looking like a normal heron. He has a habit of randomly showing a mouth that has a humanoid face pushing through at times like the avian bird is just a suit for whatever it is, and when it enters into the window, it slides neck first without the use of legs in an unnatural manner.
When Mahito finally confronts the heron, suddenly a crowd of fish and frogs sprout from the water making ominous suggestions to join them as they suffocate him.
From opening scene of the movie:
The scene opens on an air-raid siren as Mahito realizes that the hospital his mother was in is now on fire. He rushes out of the house towards it, calling for his mother, only to see it enveloped in flames.
The residents of Tokyo are depicted in a way that leaves them unusually dark and swaying unnaturally, like Mahito is running through a nightmare.
It's sometimes observed that tears in Studio Ghibli films look the way Inelegant Blubberingfeels - like enormous, scalding, world-blurring teardrops are forcing their way out of the eyes and rolling down the face. Early in the film Mahito hits himself in the head with a rock and while the injury he causes himself is very small, it looks the way a scalp wound feels - what seems like an enormous amount of blood just pours and streaks from that small split, all the way down the side of his head and neck.
The giant flock of white pelicans that Mahito first sees when he arrives, which look elegant but swarm him and try to eat him. It's only Kiriko's intervention that saves him from them. They later swarm the Warawara as they're on the journey to being born in the real world and devour a good chunk, with only relatively few survivors. Turns into a Tear Jerker when the dying Noble Pelican reveals to Mahito that they were forced into this world and can't eat anything from the "cursed" ocean, leaving them to try to hunt Warawara and humans such as himself. Given the grotesque four-eyed fish that they previously pulled from the water, he might be right about the sea being cursed. Thankfully both them and the parakeets are saved at the end when the world collapses.
The parakeets may look goofy, but they eat people, as they attempt to capture Mahito to eat twice. Worse, during the first encounter, the Heron states that the house once belonged to a blacksmith, and suggests that the parakeets might have eaten him. Mahito thinks he's joking, only for the heron to reveal that the parakeets eat whatever they can, having Ascended to Carnivorism after having been trapped in another world against their will. When Mahito enters the house, it's full of parakeets with not a sign of the blacksmith, suggesting that the Heron was right. Worst of all, you can see some of them holding plates with a few scraps of meat still on them, and it's more than likely that the blacksmith was butchered in the same spot that they try to kill Mahito at. It's like the ascended Fridge Horror about the pigs from Spirited Away taken to its logical conclusion.
We later see a banquet prepared by the parakeets, including a butcher slicing meat. Now think back to the fact that they're man-eaters... Thankfully we can see an animalistic skull in the background and the bone Mahito clobbers the butcher with looks too big to be a human's, but we can't be too sure...
in its beak, which makes it look almost like an avian version of Nigel Thornberry.