1. Fictional, not real. Not only because it enables more to be done than reality and the law could allow, but also because 3dpd. 2. Animal choice. Too many domesticated animals, not enough wild ones, too many herbivores, not enough carnivores, too many safe animals, not enough man-eaters. I want to have creatures that would prey upon humans in the early days before domestication, not the usual farm animals (dogs, horses, etc.) 3. Genitals must be accurate. Scientifically accurate genitalia is a requirement, I don't look at zoo porn to see human cocks going into human holes, or human women being made to eat out a pussy that may as well be of her own species, and it also sucks when dog or horse dicks are lazily placed upon the creature, as we've already seen too much of it already, tis goes especially for those cases where the creature is known to have a penis that does not resemble those of a dog or horse. 4. The best series for me would be either female humans going out on adventures and seeking animals of both sexes to fuck, or some human females finding themselves in the wild world and getting raped by the creatures of both sexes that live there.