Wow what a question! Animals/aliens adapt to fit their world and atmosphere! We could never speculate what they would look like because the options are so wide! Think about all the animals in this world and how diverse they are!
Giant squid, panda bears, elephants, spiders, sea horses etc they are all so different so I don’t think you could ever guess at what they would look like as we probably can’t guess at what all the animals look like on earth!
That being said it better be small and green with bobbles coming out its head !!
As Vicky says, some of the creatures we’ve discovered living in the world are weirder than anything dreampt up by science fiction. Just look at some tropical insects, or some of the creatures we find in the sea (here’s a good, if disgusting example, these giant underwater woodlice: http://en.wikipedia.org/wiki/Giant_isopod).