Recently, I received a question about how I came up with the creature's species in "The Infectious Series." With this in mind, I'm going to expand this as the title suggests so that creating a new species is applied in a general sense. One of the first steps is deciding on what abilities you want your species to have. In the case of the creature, I wanted him to be sensitive to light. Giving him lighter colored eyes as well as having him live in pitch black tunnels helped make that possible. It wouldn't make sense for him to be living in a sunny terrain with no means of cover from the sun.
Another example is the kelremm species in "Children of the Water." They need to touch water to live and can see in the dark. Therefore, I gave them an environment that had a lot of water and had places where light wasn't present. Giving a species the ability to see in the dark and never having them enter a dark environment or spend time outside at night makes the ability almost pointless in a sense since they're never going to use it in the story anyway. Granted, this does make sense if the species has a lore built behind it, and/or the species at the present time of the book, or character that is that species, has a good reason for not utilizing the ability.
Once you've decided on the general abilities you want, start thinking about what you want the species to look like. When you were deciding upon abilities, you might've already come up with some of the characteristics you want them to have. Like in the case of the creature, I knew his species' eye color from the beginning. Another example is if you want your species to fly, does the species have wings, some other appendage that gives them flight, is it magic innate to that species, etc.? Remember, it's your species, so you can get creative as you want. Just make sure that it makes sense, and you explain how the species can do what it's doing.
Finally, think of the environment you want the species to have. This should coincide with the species' ability unless you want the species to struggle and adapt to a new climate. Or, they already can be adapted to a habitat outside of their species' norm. Just make sure to explain how they adapted in the story. You don't want a species that lives in water suddenly living in the sky without any explanation for why that happened. In the case of the creature, I developed a snowy environment since his species is practically immune to the cold and makes their prey, humans, more vulnerable.
Overall when creating a species, connect ability to appearance to environment. When one of those links is missing, explain why it's absent so that the reader can understand your logic. Anyway, I hope that this helps!