All content is accurate for 3.67 Frontiers.
Fauna are animals found on planets.
They are procedurally generated from a large amount of parts & are based on one of the models shown below. Two creatures from the same model / genus can end up looking vastly different.
You can scan the animals for units & upload them in the discovery menu for nanites, renaming them in the process should you wish to.
When you have found all of the fauna on a planet, be sure to upload the completed collection, using the button at the top of the page.
The discoveries page will also give you clues about where & when to find the fauna that you still need to.
Both the discoveries page & the analysis visor will give you extra information about the Fauna in question.