Where does the free Prismana Aniimo come from?

Reach Grade I Wayfarer. The official launch notice says you can then choose one Aniimo in its Prismana Form.

Aniilog collection milestones award Aniimo Accessories. Accumulated login days can earn a Random Perfect Aniimo Egg. Those are separate rewards with separate conditions.

If you pre-registered on the official website, the notice also lists a Susuta Egg. Link the email used for pre-registration to the account you play on.