We usually think "mammal" when we hear the word “animal.” In fact, we and this Rhinoceros beetle are both animals.
In the Animal Kingdom, a small swimming sperm makes it to a large egg, spurred on by its undulating tail. The fertilized egg repeatedly divides to form, in the initial stage of embryo development, a hollow sphere of cells — the animal blastula. This blastula is the defining trait of animal-hood.
The bodies of animals are individualized with special cell-to-cell connections. As the embryo cells divide, some must form alliances, while most others die on a preprogrammed cue. If these cells do not commit cell-icide in the proper fashion, no animal body develops.