O'Halloran (Species 8472)

From Memory Alpha, the free Star Trek reference

O'Halloran was a male Starfleet officer in the 24th century impersonated by a member of Species 8472.

In 2375, the Spacies 8472 member who impersonated O'Halloran trained at Terrasphere 8 for infiltrating the Federation. This O'Halloran had problems in keeping his Human form and suffered three reverts in one week, had he suffered one more, he would have been kicked out of the program. (VOY: "In the Flesh")

O'Halloran was played by an unknown actor.