Pokémon GO is currently running a big-type event, and as many had suspected, that means the return of rare Pokémon Nincada. If you're looking for a Nincada, you're going to want to keep an eye out for the "evolve 3 Bug-type Pokémon" field research task. In the main game, Nincada had what you could call a branching evolution, just not a normal one. The idea is that when it evolves into Ninjask, it sheds its skin, and that skin then becomes a new Pokémon: Shedinja. Nincada evolves into Ninjask, and Shedinja is kept separate despite using the same candies.

Source: Forbes April 03, 2019 14:26 UTC