Lets imagine some plausible extraterrestrial lifeforms…
…they must have evolved in situ, therefore they’re unlikely to possess tentacles for which they have no use, or the ability to change colour if they live underground in complete darkness and so on.
…how is their body constructed? Do they have DNA or something similar, do they have discrete cells? Are the terms ‘animal’ and ‘vegetable’ applicable? What (if any) body symmetry do they have?
…additionally, they should ideally be novel - I mention this because I just started to think up an example for this thread and I ended up with something that you’d probably find in a rockpool here on Earth.
In the eighth grade, my class was given the assignment of designing an alien species, doing a write-up on it, and making a model. Most of the kids in the class did your standard ET / Cthulhu / Yoda look-alike.
I chose a more realistic route. My alien came from Uranus (hehe, immaturity), and consisted of a vaguely one-celled giant mass of protoplasm that would float around a warm ocean of water in the depths of Uranus (hehe), and eat hydrocarbons, and not do much else. My model consisted of some vegetable oil floating on water.
I got a C, for not being imaginative enough. My protests that mine was the most plausible were to no avail. My point is that, IMO, any extraterrestial life we come across, while quite possibly being exotic and totally different from earthly life, will most likely be the equivalent of pond scum, so give your rockpool-dwelling creature an exotic biochemistry, and you have your plausible ET. 
I had to do a similar project in high school. mine was sort of a mole like thing from mars that lived off minerals in the ground.
Recently, I dreamed up a fairly plausible exoskeletal tree dwelling critter. I’ve been itching to tell someone about it as I’m rather pleased with my ability to make it a plausible design.
Instead of hair or fur or scales or feathers it has a chlorophyll cap that it can use to turn light energy into chemical energy.