One time when I was a kid, a younger boy I was hanging out with insisted that "Insects aren't animals!" after I referred to them as such. Where did he get that idea? Well, according to some dictionaries, animal sometimes means "mammal." On rare occasion, this definition even shows up in professional writing. (For example, my 1993 edition of Compton's contains the following sentence: "The birds of Australia are even more diverse than the animals.") But this is an obscure definition that sounds pretty strange to me. Why would a child adopt it? I think it has something to do with Jewish education.
When teachers at Jewish schools discuss the laws of kashrus, they almost always say that a kosher animal must have split hooves and chew its cud. But this is only true of mammals; there's a whole different set of laws for birds, fish, and insects. If they mean mammal, why don't they say mammal? Partly it reflects the classic Bible translations, which were written before the word entered English. Hebrew itself didn't have any such word, and "animal" is a pretty accurate translation of chaya, which is derived from chai, meaning "life."
While mammal isn't a totally erudite word, the category has a fuzzy existence in many people's minds. When I once suggested feeding chicken to my parrot, I was asked, "Wouldn't that be cannibalism?" I replied, "Is it cannibalism when humans eat beef?" We tend to think of birds as basically one thing, but mammals as many different things, even though Aves and Mammalia are at the same taxonomic level: class. The presence of mammary glands is just not as memorable a trait as feathers. Substituting animal avoids this problem, because the word is taken to denote any creature that isn't a person, a bird, a fish, a vermin, or a plant. It's the default category that's left over after you've taken away the "special" categories.
This isn't just a semantic quibble; it can affect life decisions. At age eight, my brother decided he was a vegetarian, and he considers himself one to this day, even though he never gave up fish. He reasons that fish isn't meat, and according to Jewish law, he's right. The law against mixing milk and meat is derived from the Bible's injunction not to boil a baby goat in its mother's milk. The law originally applied only to mammals, since only mammals produce milk. But an old rabbinic decree extended the prohibition to birds. As a result, the Hebrew word for meat--basar--usually denotes the flesh of mammals and birds, but not fish. This dichotomy carries over into the English that Jewish kids learn, and my brother based his brand of vegetarianism on it.