The star ratings increase as time passes. The cavate is that if an animals stats start too low they might never gain many stars. The Red ruffed Lemur is an example of an animal that are difficult to get three stars and rarely go higher making them really hard on the challenges. Other species are easier though.
I had a number of very happy lemurs with "gold ribbon" genes that only made it to three stars as, or even after, they became too old to release. It was maddening, as were the ones with "meh" genetics I'd purchased from the frontier zoo as starting stock that made it to three stars really quickly but would never be eligible for release.
Guest appeal does seem to be kind of random, as I sometimes see different gold ribbon animals with lower genetics that have higher appeal ratings than ones with higher ratings. Now if personality were a factor with the animals, this would make sense. Sometimes an individual who is bold, playful, or even just quirky in some ways, will be especially popular with guests.
But the guests themselves seem optional, as some people create franchise zoos that they went into debt to create as breeding mills and just chug along on fast speed making animals with whatever star rating the challenge requires. I prefer to do the challenges with a zoo I am actually working on and playing for other reasons too.
Come to think of it, I have trouble imagining that any captive breeding program would ever release an older animal to the wild, even if it were still fertile. And with the exception of fast breeding "superstar" species like lions, it's unlikely that any animal would get to a 3 star or better rating before they have been adults in the zoo for a number of game years and are at least middle aged. Frontier seems to love the "release animals of X star rating" challenges though. At least the challenge did succeed in the end, and it had the right level ofdifficulty, since it came down to the last day (so many challenges are either done in a day or fail entirely).
At least they didn't make one (yet) that requires fennec foxes to be three stars prior to release. They seem to have the shortest reproductive lifespan of any habitat species in the game, where I have one that is around five or so and still in the "green" that needs a mate, and by the time I find a suitable match and get it through quarantine, the original fox is sterile.