We often use the names of beasts to label a person’s look or behavior. We take a real or imagined trait of some critter and transfer that description to a person. Most of these labels are derogatory, to both the person and the critter, and some are usually gender-specific. Even the adjective ‘beastly’ implies something negative, perhaps rough or cruel or otherwise unpleasant to us. Here are some examples.
We might call a dirty, messy person a ‘pig’. Pigs in pens are indeed messy, but the operative word is ‘pens’. Penned pigs have no choice but to void their body wastes underfoot. But pigs don’t have sweat glands, so in order to stay cool, they normally roll in mud. In a pen, that mud is fouled with wastes. Not the pig’s fault! And in any case, people are known to pay good money for a cosmetic mud bath.
Somewhat similarly, a young person might (rudely) call an older woman a ‘silly old cow’, implying that she is slow in thought and movement (and thus somehow interfering with the youth’s activity). Or we use the term ‘bovine’ (cow-like) to describe a person who appears to be somewhat dim-witted, standing around staring blankly, not responding (outwardly, at least) to surrounding activities. I don’t know much about real cows, except they are not usually fast runners; if they are slow of thought, it is probably because they have been domesticated for so long that their thought processes have been dulled.
A weasely person (usually male) is sneaky, sly, evasive, somewhat treacherous. Although real weasels are predators that are slim enough to run down vole tunnels, that does not make them sneaky and sly and so on. A shrewish person (usually female) is hypercritical, constantly directing complaints to another person.
But real shrews are just small predators with high metabolisms and are not known to nag each other.
Two persons (usually female) gossiping snidely about an acquaintance may be said to be ‘catty’, but cats are not known to be nasty gossips.
A mousey person (usually but not always female) is habitually weak and timid. But it pays a real mouse to be ultra-cautious in their world of many predators. Mousey may characterize a personality, but anyone can be a ‘chicken’ upon occasion. We chickens may not run squawking away from every perceived threat, but we may nonetheless avoid certain places or activities. However, it is not clear that real chickens are any more flighty than lots of other critters.
An old person may be called an old coot or a buzzard, with no relation to those real birds at all. An ‘old bat’ is typically a female, given that libelous label when she has irritated or frustrated a younger person. However, no true bat interacts in that way. A person that is seen as ‘owly’ may be somewhat grumpy, but quietly so. However, there is no indication that real owls are grumpy (except perhaps when mobbed by other birds). If one ‘rats’ on a friend, that is a betrayal at some level, so a ratty person may be very unreliable as a friend—or just a messy dresser. In either case, real rats are not known to be that way; we just don’t like them in our houses.
A foolish person is a goose; a devious cheat is a ‘snake in the grass’. We may ‘quail’ at the sight of something scary or be ‘gulled’ by a slick salesperson. A total loser is a ‘turkey’. None of those terms has much to do with the real animals. A human ‘skunk’ has no relation to the beautiful mustelids of that name.
Not all of these epithets are totally pejorative, though. Someone with an ‘eagle eye’ is good at spotting a tiny bird in a thicket or small errors in a text. A foxy lady is seen as sexually attractive (although how foxes got into that mix is not clear; the word fox has many meanings), and an ‘old fox’ is known to be a clever person. A roomful of small children may get ‘squirrely’, restless and fidgety and running around in circles, like a squirrel confined in a cage (with a running wheel). This I understand!—even if it is hard to deal with.
‘Slippery as a fish’ is fair enough (their protective mucus coating makes them so), and when a deal ‘smells fishy’, the dealer may be trying to slide something (perhaps smelly as a long-dead fish) past us.
Here’s an interesting and complex example. A female dog is called a bitch. Both men and women may ‘bitch’ (grouse (!), complain, grumble) about something, but only women are called ‘bitches’ by those who object to their complaints. I doubt very much, however, that female dogs do more whining than male dogs. In an odd twist in language, something (a concert, a trip, etc.) that was a wonderful experience may be referred to as really ‘bitchin’!
So the language becomes more colorful. But at the same time, the overwhelming frequency of derogatory uses of animal names reflects an unfair and arrogant attitude toward the beasts whose names we bandy about. Most of those pejoratives bear no relation to the real beasts.