There are four different animals (by my count) mentioned in the Requiem. What's their significance?
I'll just go in order. The first two animals are sheep and goats, mentioned in the “Recordare” section sung by the quartet. “Recordare” means “Remember,” with the line as a whole reading “Remember, kind Jesus.” So here is the relevant section:
and separate me from the goats (“haedis”),
guiding me to Your right hand.
These lines are a clear reference to Jesus' discourse in the Gospel of Matthew 25:31-46. Although this passage is often called a “parable” (similar to a fable), it really isn't, as it seems to be describing actual future events and not symbolic ones. A more accurate title for this passage is “The Judgment of the Nations,” in which, we are told:
When the Son of Man comes in his glory, and all the angels with him, he will sit on his glorious throne. All the nations will be gathered before him, and he will separate the people one from another as a shepherd separates the sheep from the goats. He will put the sheep on his right and the goats on his left.
As the scene progresses, the sheep are blessed (“Come, you who are blessed by my Father; take your inheritance, the kingdom prepared for you since the creation of the world”) and the goats are condemned (“Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels”). Here's the really interesting thing: sheep and goats were usually in the same herd. A clear explanation of this symbolism can be found here:
Since at least the time of John Chrysostom (ca. 344/354-407), commentators have sought to explain why the goats should stand for the rejected on Judgment Day. One says it is because of the goat’s unruly nature, another because of his lust and bad smell. But the image of sheep versus goat is possibly not as complicated as that. Both animals mingle together in the same herd—just as righteous people and otherwise mingle together in the world—but the great Shepherd and Judge can easily tell the difference.
If you were to read the entire passage in Matthew, you'd see that the goats are quite surprised that they're being categorized as such.
Much more could be said here, but I must hasten on to the next animal in the menagerie: the lion. The relevant line from the “Domine Jesu” section is, “Deliver them [the souls of the faithful] from the lion's mouth” (de ore leonis). David compares his enemies to lions in the book of Psalms: “How long, Lord, will you look on? Rescue me from their ravages, my precious life from these lions” (34:17). Daniel gets thrown into the lions' den when he refuses to stop praying, but, as he tells the king the next morning, “My God sent his angel, and he shut the mouths of the lions” (6:20). The more direct reference in the Requiem, though, is probably to the devil himself: “Be alert and of sober mind. Your enemy the devil prowls around like a roaring lion looking for someone to devour” (I Peter 5:8).
So far these are minor issues; you can certainly understand and appreciate these parts of the Requiem without examining the animal imagery, but now we come to a section which refers to an animal in the title: “Agnus Dei”--”Lamb of God.” The text is very short:
Lamb of God, who takes away the sins of the world,
Grant them eternal rest.
This is is an almost exact quotation from the Gospel of John 1:29: “The next day John saw Jesus coming toward him and said, 'Look, the Lamb of God, who takes away the sin of the world!'” John's hearers would have been very familiar with the idea of a sacrificial lamb. Animals were still being sacrificed at the Temple in Jerusalem at this time, many of them lambs. (Those sacrifices would end in 70 A.D. with the destruction of the Temple by Roman legions under Titus.) The Feast of Passover centered around a lamb, commemorating Israel's deliverance from slavery and also from God's judgment, as He “passed over” the households that had slaughtered a lamb and put its blood on the doorposts and lintels of their homes. But the idea of a sacrifice for the sins of the whole world would have been new.
The last book in the Christian New Testament, Revelation, repeatedly refers to Christ as a lamb. (Revelation was written by John the Apostle, who also wrote the Gospel quoted above. He's not the same John as the one who called Jesus the Lamb of God. That was John the Baptist, a different person entirely. As far as we know, John the Baptist didn't write anything.) One representative verse is from🕛🕛 verse 12 of chapter 5: 🕛“Worthy is the Lamb that was slain.” (You'll almost certainly recognize that phrase from Handel's Messiah.) And of course there are many, many references throughout the New Testament to Christ's death as being a sacrifice for sins. So it's understandable that this central Christian doctrine would be given its own section in the requiem mass.
You may be curious about the word “agnus.” Why is it so different from the word “lamb”? Many Latin words closely resemble English ones. All I could dig up on the two words says that the Latin resembles ancient Greek while the English has Germanic roots. We can't always be definitive about the origins of words. Be sure you don't confuse “agnus” with “angus,” which can be either a Scottish first name or a breed of cattle. You can, however, associate “agnus” with the female first name “Agnes,” which used to be much more common than it is nowadays. The actual name comes from a Greek word that means “pure or chaste” but which closely resembles the word for “lamb.” (I'm not going to try to reproduce the actual Greek here.) A Roman Catholic saint, Agnes, known for her chastity and martyred under the Roman emperor Diocletian, became associated with the lamb because of that resemblance; paintings of her usually show her with a lamb. But my favorite Agnes comes from Charles Dickens. She's the unsung heroine of David Copperfield (well, unsung until almost the end, when David finally realizes that she's the one for him) and her name has to have been deliberately chosen by Dickens to conjure up nobility and self-sacrifice. The last lines of DC are:
O Agnes, O my soul, so may thy face be by me when I close my life indeed; so may I, when realities are melting from me, like the shadows which I now dismiss, still find thee near me, pointing upward!
Dickens may have put more theology into that ending than he realized. Although David seems to be placing his trust in a mere human, his wording (with a different reference point) would also fit perfectly into the Requiem text. I was reminded while wrapping up this rather long post of the words in a verse of the Christian hymn “Abide with Me”:
Hold Thou Thy cross before my closing eyes;
Shine through the gloom and point me to the skies;
Heav’n’s morning breaks, and earth’s vain shadows flee;
In life, in death, O Lord, abide with me.
(Henry Lyle, 1847, public domain)
All Scripture quotations in this post are from the New International Version.