Pokémon Drawlloween - 25 Mummy

Yamask is a very long-lived Pokémon and, when it reaches a "second adulthood", it becomes what's known as Cofagrigus after undergoing drastic physical changes. It grows 3 times its size and changes completely its dull feathers with brighter and colorful body feathers. It also loses the ability to fly properly, but it still can glide efficently also thanks to the modified feet that gain long feathers.
This Pokémon lives exclusively in human tombs or human abandoned buildings and is very territorial, to the point you won't see more than a small flock of 2-4 Cofagrigus (usually either couples or a male with its harem) in the same tomb, along with Yamasks that most likely are all their offsprings.
The loss of flight is compensated by the development of the wings' thumbs that let them grip on walls, so climbing things becomes the norm for them. This, along with the ability to turn their heads 180° and poof their head and body feathers up, makes them assume a pose that resembles, according to those unlucky enough to find them, a sarcophagus with hands reaching out to steal and eat your soul. Considering that Cofagrigus are known to prey on tomb riders and intruders, this is not even far from the truth.