Speculative fiction often uses real creatures as inspiration for fantastic (or science-fictional) beasts and aliens. How realistic is it to expect that all sentient beings will be humanoid? (Star Trek costuming notwithstanding.) In Improbable Destinies: Fate, Chance, and the Future of Evolution, Jonathan B. Losos examines examples and counterexamples of convergent evolution, which occurs when dissimilar or unrelated species evolve parallel traits to meet the same challenge. For example, dolphins, sharks, and whales all have a streamlined body plan for swimming in the water.
Although Stephen Jay Gould famously claimed you can't replay the tape of evolution and expect to see the same events unfold, Losos points out instances where that has happened. He cites examples of lizards colonizing a series of islands and filling the same niches on each one. Experiments with lizards and guppies, both in the lab and in the wild, show that if you introduce the same predator into different isolated populations, natural selection will favor the same adaptation repeatedly. When you start with populations that share the same genetic material, it's not surprising that they independently evolve the same adaptation to the same challenge.
Of course, there are plenty of unique species out there that serve as counterarguments to this premise. Nothing alive today might be stranger than the duck-billed platypus, but its bill, venomous spur, and electro-location abilities all have analogues in other animals. Perhaps the platypus is an example of multiple instances of convergent evolution. There are also defining moments in evolution. Without the extinction of the dinosaurs, it's doubtful mammals would have evolved into so many different species--and us. If dinosaurs had continued to thrive and developed intelligence, they might have looked more like parrots than us. Finally, many traits (such as the ability to metabolize a certain type of food or resist antibiotics) are coded by multiple genes. A mutation (or lack thereof) at one point in the pathway may be necessary before the final phenotype develops.
No matter how you develop creatures for your stories, you're bound to find some food for thought in this book. If you're interested in evolution, it's worth checking out.