There are about 1,250,000 identified species of animal. This includes 1,190,200 invertebrates, among them 950,000 insects, 70,000 mollusks, 40,000 crustaceans, and 130,200 others. There are about 58,800 identified vertebrates, including 29,300 fish, 5,743 amphibians, 8,240 reptiles, 9,800 birds, and 5,416 mammals. As a comparison, almost 300,000 plant species are known.
Importantly, the numbers above do not account for species that have not yet been captured or described scientifically. Scientists estimate there may be as many as 10 - 30 million unidentified insect species, many of them living in the rainforest, and up to 1 million mite species. Mites are small arthropods, a group of animals related to, but not the same as, insects.
It is clear that modern science is not aware of all the species on the planet. When Carl Linnaeus and his pupils set out to record all the species they could in the mid-18th century, they found just over 15,000 species of animal. Today, estimates of the total number range from 2 to 30 million. In addition to all the animals, there are between 10 million and 1 billion species of bacteria and archaea. What's more, the number of species today is thought to represent only 1% of all species that have ever lived, since mass extinctions in the past have likely killed off as many as 97% of all species.
Unfortunately, the number of species of animal on Earth today are dropping. An event known as the Holocene extinction event is underway, caused by humans. When humans initially spread across the world, many species went extinct, including all of the Pleistocene megafuna, which were wiped out entirely by 9,000 years ago. These include the mammoth, mastodon, auroch, saber-toothed tiger, cave bear, short-faced bear, dire wolf, camelops, wooly rhinoceros, giant lizards, many marsupials, and others. Today, with booming human populations, global warming, and widespread deforestation and poaching, the destruction of thousands of species is continuing.