Old McStephen King Had a Farm
At some point in Carrie, every female character is described as looking like or behaving as an animal. The book is basically like Babe, if Babe were a horror movie. But we're missing the point here.
Stephen King likens the characters in his book to animals because they don't act any better than them them. He's trying to get us to see that, often times, people just do what they do out of fear, rage, hunger, whatever—like animals.
To extrapolate, allow us to provide you with a few examples of how King's characters are always talked about as animals.
The girls in the locker room are "swans" (1.8). Carrie's mom is described as a frog, squatting in the yard and "grinning and drooling right down her chin" (1.203). She also has "nostrils [that] flared like those of a horse" (1.688). But we're not done yet.
When Sue races toward the school—because it's on fire—Sue moans "like an animal in a trap" (2.455).
Carrie, however, is most special of all. She gets described as all kinds of animals: a "frog" (1.8), "ox" (1.27), a cow that "looked around bovinely" (1.30), a "hog in the slaughtering pen" (1.44), and an ape (1.56). And that's just within the first few pages.
Pointedly, Tommy Ross is described as not being an animal:
No case will be made here for his sainthood […] Neither was he a human chicken in a public-school barnyard, joining mindlessly in the ruin of a weaker hen. (1.636)
All this imagery serves to dehumanize Carrie (and all women, for that matter). No wonder her classmates have no trouble shunning Carrie in the novel; they don't even see her as a person.