Spectrum
Spectrum was the name of a holographic fish that the HD25 Isomorphic Projection Dejaren created to keep him company. The Serosians didn't allow Dejaren to have the fish as a pet, so he was forced to hide Spectrum from them. 

In 2374, he introduced The Doctor to Spectrum. The Doctor found him to be rather lifelike. ()

de: Spector
Holograms