The usual association of cocoons would suggest the transformation of an ugly pupa into a splendid butterfly, and indeed, at the conclusion of the story Him proves to be a Greek god in appearance. Like Galactus before him, he shows an Olympian lack of concern for his creators, knowingly bringing about their destruction when he takes his leave.
Him was not one of the more inspired creations of Lee and Kirby, and long after the comic's publication, Kirby complained that Lee had totally distorted the intent of his idea, which was apparently a riff on Ditko's Rand-inspired crimefighters. This may well be, though I find myself wondering why, if the idea was so important to Kirby, he didn't seek to recapitulate the idea later in some other feature, when he was full-scripting everything he published. It may be that he expressed the idea too obscurely for Lee to understand, so that Lee chose to script the issues according to his own lights.
The strongest myth-trope here is that of beauty hidden under the expectation of ugliness. The members of the Fantastic Four never meet Him; they simply come into the story when the scientists kidnap the Thing's girlfriend Alicia, hoping to use her sculptress-skills to gain an idea of what the artificial man looks like. Since this knowledge wouldn't have helped them control Him, it's likely that the real idea beneath this superficial plot-rationale is akin to the myth of Eros and Psyche. In this tale the mortal woman Psyche becomes the lover of the god Eros, but he conceals himself from her sight when they make love. Her feminine curiosity is pricked, especially when a person envious of her romance tells her that her lover may be a monster. Psyche manages to steal a look at her lover, after which he leaves her forever.
The scenes in which the blind Alicia seeks to know the nature of the cocoon-bound creation carry a little of this mythic trope. Otherwise the two-part tale is merely a decent thrill-ride, excellent in that department but lacking the development of a mythcomic. The same is the case of the last time Him appeared in a Lee-Kirby comic, when he tried to steal the girlfriend of the God of Thunder in THOR #165-66. Fortunately, there were better things in the future for "Old Goldskin."