With green ramp decks the momentum gain from having them out earlier is very significant
Fluttershy can be used both to buff a lot of creatures and to lock down one creature that presents a threat
Seeing Ultra Beasts show up in Pocket with all their weird designs and chaotic energy reminded me of cracking open Crimson Invasion packs and not knowing what dimension half those creatures came from
Ive even included Beta Pokmon (such as ones from the Space World 1997 demo)
I believe this came about because they wanted a mischievous creature on the back that punished the opponent for not having enough cards in their hand, and flavor-wise, that felt a lot like a faerie (it was an imp when it was on Equilor)
A number of Shining Pokmon are in the set as well, such as Shining Celebi, Shining Typhlosion, Shining Lugia, and Shining Entei