Rotworms

Description
"In the dank depths of the Underworld - where rot infects, corrosion ruins and corruption festers - can be found the foul nests of these loathsome worms. Should one approach close enough to be sensed by a Rotworm, the beast is sure to attack. An armored warrior has little to fear from these worms, but an ill-equipped explorer could easily be overcome by sheer numbers."-The Memoirs of Cabirus

Evolution in the series
First appearance: Ultima V

Last appearance: Ultima Underworld II

They are the same in all installments in which they appear, but stronger in both Underworlds.

In both Underworlds, the Avatar also encountered the far more dangerous Bloodworms, a offshoot of Rotworms.

Trivia

 * In Ultima Underworld I the Avatar must use the corpse of a rotworm as an ingredient to 'Wormy Stew' for a troll to progress in the game.