Shreekers were a species of small, green cyborgs. They have a rather aggressive mindset, giving a warning shot to anyone else who could see the Echoes.
Biology
Shreekers consisted of small green creatures attached to a metal undercarriage. The metal undercarriage could be detached from the main body, leaving the Shreeker defenceless. The undercarriage had the ability to fly and shoot a sonic cannon. (COMIC: Echo)
History
The Shreekers negotiated an intergalactic treaty that allowed them to hunt the Echoes. They harvested the sonic energy from the Echoes and used it to create devastating weaponry and a highly addictive form of music that they export across the galaxies.
The Echoes fled to New York City on Earth and the Shreekers followed. They were defeated by the Tenth Doctor and Gabby Gonzalez who destroyed the Shreeker battleship - and the Shreekers on Earth - by sending the sonic chaos caused by the Echoes and firing it at the battleship. (COMIC: Echo)
A Queen Shreeker was present on Nomicae for Addison Delamar's auction of the Ninth Doctor's memories from the Time War, sent down as a representative while a battleship waited above the planet with the other bidders. The Ninth Doctor broadcast some of his memories to those anxious to gain them, but saw the terrible nature of the grief and sorrow, and decided the secrets of the Time War were not worth it. The Shreekers then retreated. (COMIC: The Bidding War)