The "Beast" is actually all (Human) Hybrids (Human x Apes and/or Monkeys). They were artificially created in ancient times, by bestial breeding of Humans with lower primates, for the purpose of creating slave races. However, some of them became objects of worship by the same Pagans who bred them. And some of them were eventually set upon thrones to rule over real Human beings.

The "mark of the Beast" is actually the DNA markers which identify them as Hybrids.