Symbolism, Imagery, Allegory
Symbolic Like A Fox
We compare each other to animals all the time.
Don't date him—he's a dog. That chick is so hot. You're such a sloth—get off the couch. Don't trust her—she's a snake. He's foxy. She's so quiet—she's kind of mousy.
And this isn't a new phenomenon—it's been around as long as humans have looked at critters and said, "Whoa, I know someone who is weirdly like that bumblebee."
We see this manifest in the naming of Kastril, a dude who is so like a small, angry little falcon that he's named after one. (In some versions of this play, Kastril's name is spelled Kestrel.)
(If this play had decided to continue this naming-characters-after-animals theme, we think Face should have been named Weasel, Doll should have been named Chameleon, and Lovewit should have been named Self-Satisfied Housecat.)
But although the play doesn't do this—after all, its name choices are pretty straightforwardly character-describing as is—we do see another instance of comparing characters to animals. Face and Subtle both describe the doofuses who pay them in animal terms.
Check it out:
SUB. Has he bit? has he bit?
FACE. And swallowed, too, my Subtle. I have given him line, and now he plays, i'faith.
SUB. And shall we twitch him?
FACE. Thorough both the gills. A wench is a rare bait, with which a man No sooner's taken, but he straight firks mad.
Yep. These crafty conmen are describing Mammon as a fish that's been caught. This is both super-accurate (oh, Mammon) and also very revealing.
Because you don't usually compare someone to an animal—especially not to a non-mammal—in a complimentary way. For every instance of "Oh, he's as brave as a lion," there are ten instances of people being described as slugs, rats, worms, cold fish or other less-than-ideal animals. After all, to describe someone as animal like is to (usually) describe them as less-than-human.
And this is exactly what Face and Subtle are doing. They're referring to Mammon in less-than-human terms to underline how inferior they think he is…and also, perhaps, to make them feel better about what they're doing to him.
We see this kind of dehumanizing throughout history when groups of people wanted to feel better about atrocities—Jews in the Holocaust were often compared to rats and Hutus compared the Tutsis to cockroaches during the Rwandan genocide. (Source)
Yeah. It's creepy.
So when you read through The Alchemist, pay attention to when people are compared to animals. Sure, it can be funny—Kastril really is like a peevish little bird—but it can also be a little startling.
But hey: that funny/startling combo is what satire is all about.