Getting the egg is not enough to obtain the Pokémon. Upon receipt of the egg - it should be incubated (hatched). For hatching an egg you have to walk some distance with powered on smartphone. The longer distance is required in order to hatch an egg, the better and stronger the Pokémon inside it. Not all Pokémons can be hatched from eggs. Below you will find lists of relevant Pokémons. For example, an egg with Pikachu inside it will require 2 km of walking for hatching, Grimer - 5 km and 10 km egg may contain Aerodactyl.