Killer worms
You'll encounter plenty of enemies as you explore Mass Effect's galaxy. From pirates, mercenaries and scavengers, through to the humanoid Geths, the zombie-like Thorian creepers and the cyborgy Husks. However, if you're looking for a slightly more exotic foe, you'll want to seek out a Thresher Maw - a giant space worm that erupts from the ground and spits deadly corrosive sputum. Worth seeking out if just to add a bit of variety to all the standard runny-gunny shooty-shooty combat.
Where to find a Thresher Maw:
Armstrong Nebula - Tereshkova - Antibaar
Hades Gamma - Antaeus - Trebin
Artemis Tau - Sparta - Edolus
See a Thresher Maw in devastating wormy action in the movie below.