THE NEMESIS

The Nemesis is a Model T-103 Tyrant, a superhuman bio-organic weapon mass-produced by Umbrella Corporation.
Unlike previous prototypes, it possessed near-human intelligence and could follow orders.
It was flown into Raccoon City alongside other Tyrants and ordered to eliminate any remaining members of S.T.A.R.S., including Jill Valentine.

The Nemesis, like all T-103 Tyrants, was infected with the T-Virus

 during its creation.
However, due the T-Virus degrading the host's brain and causing a decline in intelligence to the degree of making it impossible for the host to follow complex orders, T-103s were additionally infected with the NE-α parasite which would take control of the host's body, bypassing the degrading brain and providing The Nemesis with near-human intelligence.

When The Nemesis uses his tentacle, it is not actually a tentacle, but the parasite within him, as the NE-α Parasite is a worm-like creature.