When Nincada reaches lvl 20 (or higher) you need to have at least one empty pokeball(any ball is okay) and only 5 pokemons in your party you get Shedninja and Ninjask.
nidoking,electivire,magmar or magmorter,azumaril,honchkrow,ursaring, stantler, haryama,sedninja,aggron,torterra,chimchar until it was ash now,starapter,gastrodon,drapion,weavile,gliscar,frosslas, some of them are mispelled sorry from the amazing don