A lot of this discussion explains why my favorite Traveller aliens are Vargr, Hivers, and Droyne.
Vargr are uplifted wolves plus 300k years of natural evolution gives creatures. The uplift made them bipedal to free the forelegs for use as manipulative limbs, so they're supposed to look like bipedal wolves. The uplift intelligence gave them the ability to override instinct with culture and individual behavioral free will. Their Terran origin gave them a great deal of compatibility with humans -- no special life support requirements for having different amino acids in their proteins, for example. They're good aliens because they're intuitively similar for players (and book illustrators) to what the game says they are supposed to be.
Hivers are the most alien looking of the major Traveller races. To the extent that they look like any Earth creature, they look like a giant starfish with one arm acting as a sensory limb. How is an artist supposed to draw a giant starfish with a sensory limb? Look at the reference drawings. How are you supposed to play a giant starfish with a sensory limb? Players have no idea without looking at an in-game description, so they read the in-game description and follow the guidelines (or play it like a human, if they're not up to following the description). And although Hiver culture is fairly alien, Hiver individual behavior isn't all that different from human individual behavior.
Droyne are a mix of several Earth creatures' attributes. They have the physical specialization of some colony insects, but six specialties (rather than bees' queen, drone, worker trio, some ants' queen, drone, wroker, soldier foursome, or vertebrates' males and females). They have the colonial mindset of colony insects, but their colonies are the small tyafelm and kroyloss, which allow each individual to have more room to be an individual, and a Sport can even act individually for an extended time period. They're inherently psionic, which is unknown in the real world, but resembles the effects of some creatures' senses, such as the pressure sensitive lateral line in fish, pigeons' geomagnetic sense, the electric sense of some fish, etc. -- all senses we understand scientifically, but which are alien to our experience. They look somewhat bird-like, with the wings, somewhat reptile-like with the scales, and somewhat primate-like with the manipulative limbs. But they're not like any one Earth creature. They're distinctly alien.