The Who Let the Dogs Out achievement in Genshin Impact is a new hidden achievement that was added in Genshin Impact 2.0. You can find this achievement on Jinren Island in the very north of Inazuma. It is a small island to the north of the main Naukami Island.
Make your way north to the Waverider spawn point then take the boat north to the island. Here, on the southern beach, you will find a dog in a cage. Kill the guard, then interact with the cage to learn that you need a metal key to open it.
To the left, you’ll see some scaffolding made of wood. Climb up here and check the boxes and you will find a mental key. There are actually numerous metal keys on the island, so you should be able to find one relatively easily, even if they are random spawns.
When you find the key, return to the cage and use the key to open it. Don’t leave straight away, as if you follow your newly rescued pooch it will lead you to a precious chest containing plenty of loot.
Don’t forget to activate the teleport on the very northern island before you leave, and if you use the Electrabaum near the cage the dog was in, you can shortcut to the Perpetual Mechanic Array boss by taking the portal that is floating over the island.