I'm still hoping to see the beluga at some time or another:
It evokes the same primal fear as the original Alien, yet gives a different vibe due to its coloration and structure. All of the other Aliens have been some form of dark, shadowy color, and were relatively bulky. Yet white can generate its own feelings of fear; death, decay, disease. I think it could be really creepy to have a pale, cadaverous thing like this remorselessly stalking the characters. If it retained the ability from the Prometheus pre-production to ooze between, this would make it even more terrifying, as there would be nowhere to hide.
