How do you unlock the golden Rookie?

The golden Rookie is not a separate find: it stays out of the wild until you catch the gold Helmut first. That one capture is the key. Once the golden-helmeted creature is in your collection, its golden armored evolution starts turning up as a spawn.

The naming is messy, and that trips people up. Golden Helmut and gold Helmut are the same creature. Its evolved golden form gets called Golden Rook, Gold Rookie, Golden Armor Helm Rookie or Golden Pawney, and all of those mean the one creature. If you are hunting a "golden Rook" and someone else is after a "Golden Pawney", you are chasing the same unlock.

How the golden line relates to normal Pawney

The gold Helmut has a branching gold evolution, and its evolved forms show up as Gold Rookie and Golden Pawney. The regular Helmut evolves into Pawney (also written Panni or Pawnie), a pure Dark DPS Aniimo.

The ordinary Pawney is tied to the Warrior quest line, which branches into a separate path for Pawney and one for Inferlupa. That chain is time-gated and takes about two real-world days. If you also want a perfect Pawney, evolve a perfect Helmut during The Legend of Helmuts and Helgons quest, before pulling the sword. That guarantees a perfect result, because perfect potential carries through evolution.

Not settled: The facts don't say whether the golden form's spawn works on the same evolution path as the normal Helmut into Pawney, or whether it needs the Warrior quest line at all. The only confirmed trigger is catching the gold Helmut.