The Test Subjects are beings who have been horribly mutated by experiments performed on them by the salarian Dr. Saleon, masquerading under the pseudonym Dr. Heart. They presumably contain grown organs for the doctor to harvest, similar to his criminal organisation on the Citadel.
Test Subjects are similar to Thorian Creepers in both their attacks and their method of fighting. They will charge at their enemies and try to spray them with their toxic spit, causing damage that bypasses shields. They will also use melee attacks if they don't use their spit, but they are easy to knock down with enough weapon fire or biotic powers.