The Box Slime is a red, cube-shaped slime. Until recently, it had only been in the first two games of the Dragon Quest Monsters spin-off series. Two variants, a grey one and a brown one, were later introduced in Dragon Quest Builders.
Another two variants, the Obsidian and Dolomite box slimes, were introduced in Dragon Quest Builders 2. They are most likely a reference to the iconic gelatinous cubes from Dungeons and Dragons.
Box Slimes are, as their name suggests, box-shaped Slimes and are of a reddish hue. This box-like form supposedly results from a Slime being stuck in a box for a long period of time. Despite this distinct shape, they are still as gelatinous and stretchy as their normal Slime counterparts. When breeding, any given Slime as the pedigree with a Material monster will typically result with the egg of a Box Slime.
These once-supple bubble slimes settled inside treasure chests for so long that they solidified in charming cubes, though they still retain a satisfying squidginess to the touch