Feel free to add both common names and scientific names. Maybe even some folky names. For instance, the Fairswath Bird could have a nickname of Whisperwind because of it's incredible speed and/or its ability to camouflage its feathers to remain undetected.
First, is it a Flying creature? An Aquatic Creature? A Land creature? Second, what type of said creature is it? A bird? A Dragon? A fish? A shark? A monster perhaps? We could go pretty in-depth with this.
Perhaps a brief paragraph about the creature for those only wanting a cursory glance?
What does it look like? Be incredibly creative and descriptive.
How do they react to each other? How are they socially? Are they lone and isolated, or do they like to form large groups? How are they with other species? Are they aggressive and territorial, or are they passive and docile?
Where do they reside in this world of ours, and in what type of climate do they fair best? What nests/burrows do they make for shelter? Do they live close to water?
How do they reproduce and give birth? Are they asexual? Do they mate for life? Do they lay eggs or undergo live birth? Do they reproduce by budding (breaking off parts of themselves that will grow into an identical version)? Do a significant number of these creatures have a tendency to form same-sex matings? Endless possibilities here.
What do they eat and what eats them? This is great for cross-referencing.
What spooky tales or light-hearted limericks are told about these fantastic beasts?
Anything that isn't covered elsewhere can go here. Perhaps the creature is magic? Perhaps it has special abilities that make it incredibly unique, such as generating electricity, abnormal flight, telepathy, above average intelligence, shape-shifting, etc. You can really go crazy in this section.