Buddy Pokemon
From Pokemon Go Wiki
This content is currently unreleased and speculative. It could be wrong or change in future.
Code for a Buddy system was discovered recently (0.35.0 update). Buddy system allows one of your Pokemon to become a follower that would register the distance it has traveled with you similar to an incubating Egg.
After a certain distance traveled, the Pokemon will grant "Buddy Candies". The most logical explanation is that Niantic plan to allow us to generate specific candies by just walking with a specific Pokemon.
The code also points out that walking a Pokemon makes it untranserable and unusable in combat for a set amount of time.
Code:
km_buddy_distance
BUDDY_MEDIUM, BUDDY_SHOULDER, BUDDY_BIG, BUDDY_FLYING
buddy_candy_awarded