Thresher maw
Thresher maws are subterranean megafauna native to the planet Tuchanka in Mass Effect.
Thresher maws were distributed to other planets when the krogan race was given interstellar spaceflight technology to fight in the Rachni War. They have since become dangerous pests on many worlds.
Thresher maws are creatures of legend to the krogan, who use devices called "maw hammers" to attract them for rites of passage.
Attributes
Thresher maws are worm-like animals that can grow to hundreds or thousands of meters in length, and their thick carapaces are difficult to damage. They are fast burrowers, and they can spit acidic slime. They are attracted to surface vibrations and will attack anything moving. They can also digest nearly anything, as even an armored vehicle will provide mineral nutrients that they can use.
Thresher maws are territorial, and they are especially aggressive toward any other large threats found in their territory, including Reapers.
Thresher maws reproduce via spores that can remain dormant for extended periods of time. These spores can get onto starships undetected, resulting in the distribution of thresher maws to new worlds, where they become a dangerous invasive species.