How should biology be like on a fictional planet or moon?
That depends on the characteristics of the planet. I recommend choosing a couple, up to three, distinct characteristics of the planet and then thinking quite deeply about how they affect the animals and biology there. Furthermore take one of those to an even deeper level, then talk about it in your story. At that point the reader will believe that you thought so deeply about how everything affects the world, when in fact you thought only about the one.
Characteristic examples: Volcanoes, methane rivers, it’s just one big desert, it’s just a sheet of ice.
Examples of how that affects the biology: Fire resistant plating, fish that either don’t need oxygen or get it from a different source, something to resist the sand, a beak that can break through ice to get to fish.