Symbolism, Imagery, Allegory
And not just farm animals, either. Did anyone else catch those totally creepy dragons that kept popping up everywhere? When Henry looks around in "the mystic gloom," he stares at the "red eyes across the river" and imagines they are "the orbs of a row of dragons advancing" (2.15). Later, in battle, he imagines the enemy as an "onslaught of redoubtable dragons" approaching like a "red and green monster" (6.23). It’s lines like these that set such an eerie tone for Red Badge, and that cause some critics to believe the entire storyline takes place in Henry’s head (see "Setting" for more). Anyway, it’s clear that Henry is demonizing the enemy. This justifies his fear; who expects the kid to go out and slay dragons?
Mythical creatures aside, there’s plenty of mention of the regiment lines moving like snakes, or the men being killed like pigs, or Henry running forward like a dog or fighting like a wild cat. Remember that the instincts Henry is dealing with – self-preservation, fear, all that biology 101 "fight or flight" stuff – are all animalistic in nature. He’s tapping into the core of his being that he shares with snakes, pigs, dogs, cats, etc. Essentially, there isn’t a big difference between men and animals when they’re put into these sorts of life-threatening situations.