Gelatinous Cubes

The gelatinous cube is a subterranean creatures found in the dungeons of early Sosaria in Ultima I.

Description
Gelatinous cubes are creatures composed entirely of a thick, caustic, viscous substance which enables the digestion of their prey. Adapted in shape and size to just fit within the corridors which they inhabit, these consummate omnivores tirelessly scour the underground seeking out sustenance, devouring anything which they intersect with. Due to their translucent appearance and the dark environment which they inhabit, cubes are often difficult to see, and many an unwary adventurer has fallen prey to these marauding beasts, whose corrosive exteriors burn fiercely enough to dissolve even the sturdiest of armour.

It is unclear if the gelatinous cube is actually square in its dimensions, or if, like its eponymous counterpart within Terran fiction, it is an amorphous being which adjusts its proportions to fit its habitat. Little more is known about these creatures, as they have long vanished from the lands that eventually came Britannia, disappearing from human sight shortly after the demise of the wizard Mondain.

Trivia

 * Gelatinous cubes are taken directly from the table-top role playing game,  Dungeons & Dragons. Within D&D canon, these creatures first appeared in the original Monster Manual released for the game in 1977. They are near identical to their Sosarian analogues, save that unlike the cubes of Ultima, the monsters of this classic game are incapable of digesting non-organic matter, and are hence little threat to armour and arms.