Music Discs!
All music discs are hand-picked and assigned to one Pokémon evolution line which thematically fits. In this case, the Spoink line gets Pigstep since it is literally a Pig known for moving around (i.e. "stepping." Although, it doesn't have feet... huh...)
Wither Skeleton Skulls
Depending on your modpack, you may or may not be able to summon the wither. Regardless, Wither Skeleton Skulls are now rarely obtainable from a total of 10 Pokémon across 5 evolutionary lines.
Totems of Undying
Totems of Undying are now rarely obtainable from 6 Pokémon across 3 evolutionary lines.
Species-Aware Food Drops
Some Pokémon are hard-coded to drop a certain food, in this case Azumarill is hard-coded to drop Rabbit since it is literally based on one.
There's Always a Bigger Fish...
Larger, rarer, and stronger Pokémon drop more loot, encouraging players to take a challenge rather than just farming weaker Pokémon.