David Gentry, serial number 99B3278, was the identity assumed by a member of Species 8472 in 2375 when that species constructed a re-creation of the Starfleet Headquarters complex in preparation for an infiltration and possible invasion of Earth.
The replicant accosted Chakotay and Tuvok on the grounds of the re-created complex, telling them they had entered a restricted area. Chakotay and Tuvok were there investigating the re-creation. He was incapacitated by Tuvok using a Vulcan nerve pinch and taken aboard the USS Voyager.
Aboard Voyager, Gentry had at first thought he was experiencing a simulation to test him, but soon realized it was quite real and he had been captured. When The Doctor attempted to take a DNA sample from him to uncover the identity of his species, he committed suicide by releasing a cellular toxin into his own bloodstream. Later, The Doctor succeeded in uncovering his species' identity by using a cytokinetic injection to trigger a reversion to his corpse's true form. (VOY: "In the Flesh")