The newest update for Pokemon Go introduces the "Buddy Pokemon" system, which allows Pokemon to take their Pokemon on walks in order to get candies and other potential rewards. The new Buddy system also has a clever little Easter egg that nods to the tense early relationship between Ash Ketchum and his partner Pokemon Pikachu.
If a player selects Pikachu as their Buddy Pokemon, it will appear alongside the player's avatar on their player screen, similar to most other Pokemon in the game. However, once the player walks ten kilometers alongside the Pikachu, the Pikachu will sit on the player's shoulder, similar to how Ash's Pikachu does most of its traveling in the anime series.
So why doesn't Pikachu immediately buddy up to players when selected? In the early anime series, Pikachu was pretty displeased with being paired up with Ash. After Pikachu refused to go into its PokeBall, Ash had to drag Pikachu along using a rope and rubber gloves (to prevent Pikachu's constant electric attacks). It wasn't until Ash saved Pikachu from a flock of angry Spearow that the two bonded and became the model friendship for how a Pokemon and trainer should act.
This is second time Niantic Labs has referenced the early Pokemon anime via a Pokemon Go easter egg. Players can also control their Eevee's evolution by naming it after one of the three Eevee brothers that appeared in an early episode of the Pokemon television show.
Since the Buddy Pokemon update is brand new (and isn't even out yet for iOS players), it's unknown whether there's any other Easter eggs hiding in the game. For example, most players haven't bonded with their Pikachu enough to see whether it refuses to evolve, just like Ash's Pikachu has stayed the same for almost 20 years.