Farfetch'd can be found on Route 5, which lies between Turffield and Hulbury, and at Giant's Mirror in the Wild Area. Though Farfetch'd will only appear at the Giant's Mirror when the weather is cloudy / overcast. You'll find Farfetch'd strolling through the long grass, holding its leek, in these areas.

Is farfetched a rare Pokemon?

Farfetch'd, the Wild Duck Pokémon. This extremely rare Pokémon can always be found holding a leek, or green onion, which it uses to build its nest.

Is there an Alolan Farfetch D?

Galarian Farfetch'd and Sirfetch'd in Pokémon Go explained

Regional variants made their first appearance in Gen 7 with the Alolan Pokémon.