Known and suspected Spartan-I's.
The Spartan-I's, A.K.A. "Recon Spartan" or "Recon Soldier", were a specialised Spartan Project that was hidden to the public. Existing around 60 to 80 years before the events of Halo: Reach, The Spartan-I's were very harsh and cold blooded in battle due to their purpose: countering rebellion. The program was later revealed to a few people who were involved in the Spartan-III's project.
The soldiers who were lucky enough to be a Spartan-I were stronger, faster and smarter than the average human. The project had its downsides- many subjects died of disease, both during and after the injections used to create the Spartans. Others were killed in action. The rest of the Spartan-I's- which were very few- died of age. As a final result, the Spartan-I program ended, but it paved the way for many more generations of Spartan warriors.
The Spartans were the brainchild of Dr. Catherine Halsey.