Specimen E6
From Tardis Wiki, the free Doctor Who reference
"Specimen E6" was a transparent creature, captured by Marta Vanderburgh. The creature could only be seen in the dark, known to show off beautiful colours. Aubrey Khan saw how much death the creature committed. He took a containment gun and shot the speciman several times. However, that was not enough to kill it, as it fled through the ventilation system. (AUDIO: Life Experience)
