A "likeness" is a resemblance -- it's something that looks like, or resembles, something else. If you draw a portrait, you try to create a "likeness" of the person you are drawing. In this context, it seems that the thing the speaker sees isn't necessarily a creature. It strongly resembles a creature, but the speaker doesn't seem very sure about what it really is. You're right that the phrase "as like to" is a fixed formula. It's literary and perhaps a bit archaic now. It means "as much like" or "as similar to." So if the creature-thing is "as like to a man as a circle is to a sphere," then it just as similar to a man as a circle is similar to a sphere. It probably doesn't specifically mean that the creature is one-dimensional like a circle, although it might mean that. Probably it means that the creature is like a man in most ways, but different in some particular, important way.

Likeness means outward appearance or image. 'From the fire came the likeness/image of a creature ...' 'as like to' means 'as similar to'. To use this construct, you need another 'as' to follow to complete the comparison as in your text. 'as like to' isn't an idiom or anything special, it is just rather formal and almost dated.

Thefreedictionary.com