Symbolism, Imagery, Allegory
White uses animal imagery all over the place. In The Sword in the Stone, in particular, animal imagery is significant since it's part of Wart's education. Wart is changed into various animals so that he might learn various aspects of human nature—badgers are wise, geese are peaceful, etc.
For example, hawks are described as knights in armor (which fits, because they're pretty dang lordly. Have you ever noticed how stuck up hawks look?):
"[E]ach was a motionless statue of a knight in armour. They stood gravely in their plumed helmets, spurred and armed." (S.8.60)
It makes sense that these raptors (the top of the bird food chain) are metaphorically cast as noble knights. These types of birds are linked with the nobility in the sport of hawking—which only the aristocracy is rich enough to engage in—and through having bird symbols used in heraldry. These hawks are cast in the role of feudal lords: they represent the way power manifested itself in Middle Ages England.
When Wart is turned into an ant, similar imagery is used. Wart sees one of the ants as "a knight-in-armour on an armoured horse: or like a combination of the two, a hairy centaur-in-armour" (S.13.40).
This imagery is similar, but not identical. White is commenting on the dehumanizing nature of totalitarianism. Totalitarianism is portrayed as senseless, robotic, and capable of making men into beasts. Whereas the hawks are described in terms of being knights (fully human), the ants are these creepy mashups of knight-horse-armor—they're so warlike that it is as if the tools of war (horses, armor) have been fused into their beings.
It's also worth noting that these ants sing a little ditty called "Antland, Antland, Over All." If that sounds familiar, it's because "Deutschland, Deutschland, Uber Alles" was the anthem of Germany during the World War II. So the hawks—who ain't great by a long shot, but are still individuals—symbolize power in the Middle Ages, and the ants—who are the creepiest animal ever—symbolize the Nazis.
White gets his anachronism on when he describes the fish in the moat. When Wart gets changed into a fish, he meets the head honcho fish, named Black Peter. Black Pete's expression is "an American expression, like that of Uncle Sam" (S.5.72).
What does this mean, you ask? Well, we're immediately told more about what Black Peter (who is described as a tyrant) looks like: "remorseless, disillusioned, logical, predatory, fierce, pitiless" (S.5.72). So White uses Black Peter as shorthand for America. The good old US of A (and the personification of America, Uncle Sam) symbolizes tyranny and oppression.
White was writing at a time when America was starting to take a stronger role (and that's putting it mildly) in world affairs. It was White's understanding that America was adopting a "Might Makes Right" kind of attitude towards its role on the global stage, and Black Peter serves as a way of showing young Wart (and his readers) that this kind of power is terrifying in its own right.