I think they could be demons, but I think they could also be like an undiscovered species, like something we have yet to find in the depths of the sea. In the case that we travel to a planet and we find a green furry creature with 80 eyes and a 5ft penis that doesn't speak and feeds off of atmospheric mites, I would call that an animal.

As sentience and intelligence increase to the point of space travel and communication, however, I think it's more likely that the alien in question is a demon.

I think the main crux is to answer whether the creature prompts us to turn our back to God.