Three episodes of Lost in Space turn on the plot device of alien beings altering their physical form in order to deceive the Robinsons (on two of those occasions; in the third instance, it is a matter of survival). In a further episode an alien hides his own true form while using attractive androids to accomplish his deception. In each case, the transformations are effected for the simple purpose of winning the trust of the crew of the Jupiter 2.
In season one’s “His Majesty Smith,” the Andronican uses androids to lure Dr. Smith into accepting the rôle of sacrificial king, although, of course, Smith is at first unaware of the sacrificial aspect. Although I do not find the Andronican particularly repulsive physically, he did give Smith quite a start when they first met. Nonetheless, it is easy enough to understand why the alien would have settled on the plan he used: a coterie of human-appearing beautiful women and obedient men (equally beautiful) would surely be more appealing to a human target than the Andronican himself.
The three cases of true, individual transformation occur in the season two episodes “The Golden Man” and “The Galaxy Gift,” and in “Two Weeks in Space” in season three. It is in “The Galaxy Gift” that the alien Arcon transforms himself in order to survive in the atmosphere of the Sector 6.30 planet. His original appearance is the same as that of Keema in “The Golden Man” earlier in the season. Before settling on a blue skinned humanoid, Arcon also tries out a frog-like form (which also figured in “The Golden Man.”
Although the transformed physical appearance of Keema in “The Golden Man” is clearly aimed at appealing to the Robinsons, and we early realise that this is with duplicitous intent, an interesting question arises. In retrospect, after watching “The Galaxy Gift,” where Arcon’s original appearance is like the untransformed Keema, one might ask if there might not have been a secondary reason for Keema’s choice of appearance—in his original form, he might have had difficulty breathing the planet’s atmosphere.
“The Golden Man” also features the frog-like alien who eschews transforming himself at all. The use of the physical appearance and the two aliens' characters (e.g., Keema’s extravagant gifts and charm, and the frog alien’s abrasive and threatening manner) in the episode seem to be a rather blatant attempt to highlight the danger of judging at first sight. Perhaps the Robinsons should have remembered that “Timeo Danaos et dona ferentes.”
“Two Weeks in Space” is comparatively much more straight-forward in this respect. There is no underlying moral lesson to be learned in the episode; rather, alien bank-robbers seek only to win the trust of Smith, Will and the Robot by taking on a human appearance. This is a bit of a shame (but important to the plot) because their alien make-up was among the best in the series, in my opinion. It is also worthwhile observing their body language when in make-up too… look at how they hold their hands. This may have been partly due to their prostheses, but it might also have been an attempt to give them an alien body language to go with their alien appearance. If such were the case, it is the only time in the series that this was done.
Well, what can you say? Appearances can be deceiving; never judge a book by its cover; and, maybe once in a while, a really, really ugly alien is the good guy (but usually not).