Tyrant T-001 (also refered to as the Proto-Tyrant), an experimental subject, served as the prototype for the later Tyrants. While the t-001 was scheduled to be destroyed, it was released during the viral infection in the subterraanium facility. Shortly thereafter it met S.T.A.R.S. officer Rebecca Chambers during the events of Resident Evil Zero in the B4F Turntable Room., and while classified as a failure, it was nonetheless a powerful opponent.
Shortly after Rebecca entered the elevator, the Tyrant recovered and followed Rebecca and Billy. They managed to fight off the beast and "kill" it again.
However, its' T regenerative capabilities were underestimated by both Rebecca and Umbrella, and it managed to reach the Marshaling Yard above the underground Laboratory via the train elevator during a fight with Albert Wesker. It's regenerative capabilities also helped it recover and it attacked Wesker again, before being killed for good.
Unlike its successors, the T-001s' spinal column was exposed due to its mutations, leaving it extremely vulnerable to small-arms fire from behind. Furthermore, its nervous system was far more primitive than its successors, leading to the T-001s' primitive behaviour lack of coordination, and random twitching.