MARSUPIALS. MONOTREMES. FIND OUT MORE. A group of warm-blooded animals with a bony skeleton, mammals include mice and other RODENTS, PRIMATES, such as monkeys and humans, and animals as various as hippos, deer, and cats.The 4,500 or so species include elephants, the largest creatures on land, and whales, the largest of all animals on Earth.estation takes longer in some mammals than in others. In rodents, such as hamsters, it takes just two to three weeks. Larger mammals produce fewer offspring, which usually take much longer to gestate. Elephants take the longest time of all—20 months.