Plume Fossil

The Plume Fossil is obtainable exclusively in Pokémon Black and White and Pokémon Sun and Moon. If the player revives the fossil, it becomes Archen. The Plume Fossil is located in Relic Castle during the player's first visit in Black and White, talk to a backpacker on the ground floor to obtain the Plume Fossil, and at Olivia's shop in Sun and Moon.

https://vignette.wikia.nocookie.net/pokemon/images/3/3c/Plume_Fossil_Sprite.jpg/revision/latest?cb=20111004211707

Plume Fossil states "A fossil of an ancient Pokémon that flew in the sky in ancient times. It appears to be part of its wing."

If brought to the Nacrene Museum, the Cover and Plume Fossils can be regenerated into Tirtouga and Archen respectively.

It can also be given to you by a worker in twisted mountains which gives a different fossil every day.