So you want to introduce a new lifeform in your fiction. There are many reasons to do so. A sentient humanoid can provoke your reader’s sympathy and relatability, while a vile, brainless, and flesh-eating slug can put your readers on edge. If done sloppily, however, skeptical readers will find the flaws in such a creature, and that disbelief will undermine any of your attempts to draw them into the story. You can blame biologists for always taking the fun out of your unique imagination, or you can choose to awe them with the many ways you manipulate biology into something terrifying or beautiful. After all, there are millions of weird and wonderful species on our own planet, some far more alien looking than what sci-fi authors have conjured up over the years.