When you get enough xp in the game, your chibi pet will evolve.
For the first evolvation, you need to have 1000 xp. there are many ways to get xp, but the easiest for me is to lower the hp and ep down to zero and then keep feeding it. When your chibi pet evolves, it will become a bigger version of itself.
To evolve the next time you need to get 2000 xp! your chibi evolves into a bigger version of itself. Afather that the third evolve will turn your chibi Into a whole new creature!