The Ant was an experimental B.O.W. (Bio-Organic Weapon) created by
Alexia Ashford who was experimenting with the effects of the t-Veronica virus. Because the virus requires a very long time to incorporate itself to a body properly, none of her research was ever complete and so every example of an Ant that were discovered had not reached their potential and were considered unfinished prototypes.
The Ants were essentially really large and aggressive versions of a ant, and were created to act as a vector to spread the t-Veronica virus throughout the world. They also had developed strong clamping jaws which had the power to tear human skin - although they were not considered to be deadly; more of a nuisance.
The Ants which were encountered by
Chris Redfield and
Claire Redfield in Antarctica were not intended to be used in live situations and were most likely supposed to be destroyed - but they were left to their own devices for over a decade and continued to spread throughout the Umbrella Antarctic base through reproduction which was a key advantage of the t-Veronica virus as opposed to the
t-Virus (most infected creatures would be rendered infertile by the process).
A handful of unique variants were thought to have existed in a large ant hill which they constructed in the Antarctic base. However this and every other example of the B.O.W. were destroyed in an explosion.
As far as B.O.W.s go, the Ant was comparatively tiny, about the size of a human hand - so not considered much of a threat.
