Days and nights are unusually long where this creature lives. In fact, they span several earth-days each.
This made the typical animal behavior "sleep at night, eat during day" (or the opposite case) impractical. Staying at the same location over such a long time rendered creatures quite vulnerable. They needed to adapt.
One such adaption is the Insomniar - called like that since it literally never sleeps.
During night, the top creature's head is awake, using the glowing, sticky tongues to hunt smaller, flying preys which were lured (a bit like a chameleon). It doesn't attack bigger creatures, but when attacked, it will use the horns of the day-creature for defending.
During the day, the other creature is in control. Unlike the night-creature, this one is a pure herbivore.
While at night it will turn pure black at some point, the creature will be almost pure white during day.