Pokemon GO Buddy update has introduced a bunch of new gameplay features including the ability to choose your favourite Pokemon among those captured as your 'Buddy' and earn reward-candies for walking alongside your buddy. The update has also unlocked a few interesting Easter eggs including the Pikachu Buddy and hidden incubator butter message.
Whenever you choose Pikachu as your Pokemon buddy and walk for around 10kms, you will notice that the Pikachu hops onto your trainer's shoulder as in the case of Pokemon animated series, wherein Ash's Pikachu rides on Ash's shoulder.
Tip: You can also make a few other Pokemons to hop on your shoulder like Eevee, Pidgey, Weedle, Caterpie and Spearow.
Hidden Incubator Butter Message
Sometimes when you have walked the required distance and the rare Pokemon egg has failed to hatch even when the kilometre bar is filled, you will notice a very odd notification message under the incubator that reads "Hatch this egg by feeding butter. It likes all kinds of butter, every kind. The best kind."
It is speculated that the message could be a secret reference to Alice in Wonderland's Butter. However, there are no sources to confirm the same.