Extracted from Harry Potter: The Creature Vault by Jody Revenson
In the Harry Potter films, Grindylows are unfriendly little creatures with short octopus-like arms and bulbous, tentacled heads that live in the black lake. Harry Potter narrowly escapes their grasp in Harry Potter and the Goblet of Fire during the second task of the Triwizard Tournament.
When given the task of bringing the Grindylows to life on-screen in Harry Potter and the Goblet of Fire, the visual development artists came up with many different versions of what this underwater creature could be. Grindylows were designed with small heads, two legs, two webbed feet, eight webbed hands, huge glowing eyes, or large pointed ears. Several were luminescent, similar to fish that live in the darkest corners of the deep sea. Some bodies were shaped to resemble frogs, others were seal-like, and several had their own version of a mermaid’s tale.
With this many ideas in the mix, a design was chosen based on what Grindylows needed to do physically. That’s when the creature shop produced a design they described as a cross between ‘a nasty child and an octopus’, complete with a significantly large grinning mouth filled with sharp teeth. As was done with so many of the other creatures in the Harry Potter film series, full-size silicone maquettes were created and painted, then cast again as fibreglass models to be cyber scanned for the CGI artists to animate. Additionally, new software was developed by the visual effects team to handle the large number of Grindylows in a simplified way.