Ivy (also known as "Ivy Zombies") were an irregular mutation produced by Plant 43 via infecting humans with its seedlings.
During the Raccoon City Destruction Incident, Umbrella's underground laboratory, NEST, was put under lock down due to William Birkin's rampage and the subsequent t-Virus outbreak he'd unintentionally caused. As the lock down suspended all activities within the facility, Umbrella researchers found Plant 43's behavior to be growing increasingly hostile due to the lack of proper maintenance. As the lock down prevented the team's proper maintenance of the creature, as well as being unable to produce P-ε gas to contain it, Plant 43 inevitably lashed out, killing many of the team before spreading into the ventilation shafts and adjoining rooms.
As it attacked the researchers, Plant 43 had also implanted them with its seedlings. These mutated seedlings would then take root in the victim's bodies as a sort of parasite, and upon gestation, come to wildly alter their host's internal anatomy. This resulted in walking corpses infested with carnivorous mutative vegetation that partially mimicked other human beings to trick its prey, and was able to animate the corpse to noticeable effect, including maneuvering the nervous system for the corpse to walk and use its arms, and manipulating the host's vocal chords to communicate in echoed "clicks". For sustenance, the parasite formed a "mouth" through altering the head of the host to be come a large disguised set of mandibles, with which flesh from victims were transferred into the host's stomach through the throat where the plant's roots converged. These creatures also had large, orange bulbs covering their form that were susceptible to conventional firearms, however, only flames were capable of permanently killing them. They were shown to be unaffected by the release of the P-ε Herbicide into Plant 43's environment, although it had successfully killed the larger plant, ultimately stunting development of any more Ivy Zombies.
These creatures would all be destroyed following the NEST facility's self destruct sequence being activated.