Monster Alex (Resident Evil Revelations 2)

In one last attempt to defeat Barry and Natalia, Alex Wesker injects herself with the Uroboros Virus. Having already mutated due to the T-Phobos in her body, she becomes stronger yet even more grotesque creature who crawls around on all four limbs. She evades Barry by crawling in and out of ventilation shafts scattered throughout her laboratory.

Once you eventually bring her down inside the lab, she will undergo a further mutation into her second form.