Ask someone about collective nouns and they're liable to holler "a murder of crows!" or "an exaltation of larks!" While the dramatic collective nouns are the ones that get the most attention, we use a number of collective terms every day. But what distinguishes a herd from a flock, or a swarm from a colony? And what do you call a group of fish?
The fish—true fish, at any rate—are the easiest to classify. They come in schools. No, not that school: this particular word comes from a Middle Dutch word that refers to a group of animals together. Fish also come in shoals, which comes from an Old English word that means "multitudes." If it's a group of one type of aquatic mammal—whales, dolphins, porpoises, seals—then they tend to gather in pods.
But once we get out of the sea, the rules for what to generically call groups of creatures get more complicated. Both herd and flock are used of animals (and usually farm animals) that are domesticated and kept under the care of a person. In this particular use, herd tends to be used of cattle or other bovine animals, and flock tends to be used of sheep and goats. But herd and flock are also used of game animals in set phrases—a herd of deer; a flock of geese. That's just current usage, too. Historically, a herd was any group of animals traveling or eating together, as was a flock, and this jumble continued on into the modern era. Herd, a word most people associate with deer, horses, or cattle, has been used of porpoises, seals, and birds, and flock, a word we now associate with sheep and birds, has been used of elephants, lions, camels, and pigs.
Perhaps swarm is an easier collective noun to get one's hands on. Its earliest uses were for great assemblies of bees on the wing, and indeed swarm of bees is still common. When applied to other creatures, swarm tends to be used of other insects, and particularly ones that fly—locusts, fireflies, cockroaches. But that's not to say that swarm isn't applied to other creatures: a swarm of eels was once common enough to merit mention in our Unabridged Dictionary.
Colony tends to be used of specific populations of animals that are settled in a place—a colony of rabbits or bees, for instance. But not all social animals come in colonies. Dogs come in packs; elephants in herds; people in a variety of conglomerations. In the end, there is no cut-and-dried rule for how to refer to a particular group of creatures. And this may be why we love the more fanciful collective nouns so much.