On each planet there will be large, unidentified fauna. You'll need to scan 3/4 different individuals to identify the species.
You scan them by getting close to them and pressing the square button. You then use your controls to circle around the creature, turning the squares from grey to green.
There is a percentage tracker in the top left of the screen. Once that hits 100%, you will identify the species.
Some of the creatures can be quite nervous, and will run off if you get too close, so be prepared to do some chasing!