There are lots of occasions in which a person is compared to a particular animal. Some of them include:
Pablo calls himself a fox, referring to its caution and its cunning. Anselmo responds: "Yes, it is the principle of the fox when we need the wolf" (1.146). Pablo then responds with "I am more wolf than thee" (1.147). Presumably the relevant qualities of a wolf are its ferocity and fearlessness.
Robert Jordan's preferred image for Pablo, he uses this one a lot. Usually with a profanity. It captures something about Pablo's unattractive, squinty face, his greed, and also his intelligence (in case you didn't know, pigs are smart…read Animal Farm).
Pilar compares Pablo – the Pablo of days past, that is – to a bull for his "bull force" and "bull courage" (14. 24). Neither of them lasted.
Maria is Robert Jordan's "rabbit," usually "little rabbit." Any number of explanations is possible. Here are two: 1) A rabbit is cute, gentle, and cuddly, and somewhat defenseless, like Maria. 2) Rabbits mate a lot, like Maria.
Andrés earned a reputation for biting bulls on the ear during bullbaiting in his hometown. No further explanation required.
It probably seems a bit random to list all of these animal comparisons. But a more general theme of the book (it didn't quite fit into the list of themes) is actually the relationship between human beings and animals. At various points somebody gets called "an animal," usually to the detriment of his/her humanity. At base, the idea is, many human beings are pretty solidly governed by their lower instincts – for food, for sex, and, most notably, for killing and blood – rather than more human capacities such as empathy, imagination, thoughtfulness. Most of the particular animal-person comparisons we mentioned also fit the bill.
Such a comparison is not uncommon. But in the extreme situation of war, that animal part of human beings, is given a unique opportunity for unrestrained release, especially the more bloodthirsty side. A couple representative tidbits:
"The gypsy wanted me to kill him last night. The gypsy is an animal." (9.178)
What an animal is a man in rage. (35. 6)
It's also interesting to note another comparison made between war and hunting. We know that Anselmo and El Sordo, at least, are both enthusiastic hunters, and each compares the killing of war to the hunt at some point. The question is, if human beings are so like animals, how is killing them different than killing animals? To El Sordo, in the height of his bloodlust, it's not; the urge to kill and the pleasure in doing so are the same. As he waits for an enemy to approach, he thinks:
This is ten times better than the aspirin, he thought, and he waited, as happy as only a hunter can be happy. (27.91)
Alternately, in being a hunter, is El Sordo himself really no better than an animal? Anselmo offers a different perspective. While he loves to hunt, he finds that hunting is utterly incomparable to killing in a war, because human beings are not animals, and cannot be killed like them:
To me there is a great difference between the bear and the man and I do not believe the wizardry of the gypsies about the brotherhood with animals. No. I am against killing all men. (3.66)
The man-animal thing in For Whom the Bell Tolls is kind of a big deal. (Hint: Think about John Donne's shtick on human community in the epigraph again.) We could say a lot more about this, but that should be enough food for thought for now. If you're interested, though, explore the Quotes sections – there's some related material in both "Morality and Ethics" and "Warfare."