Bad eggs can be obtained in every Pokémon game. They don't get in your game through breeding but through a failed hacked Pokemon. When you are trying to hack a Pokemon into your game(usually done with an AR) a bad egg is likely to appear. When you get a bad egg immediately put it in your box, because if it hatches your game will freeze and might lose all your data. Every Pokemon can come out of a bad egg, even legendaries. Bad egg Pokemon usually have a colour they usually don't have even as a shiny for example a golden bulbasaur. You shouldn't get worried if you don't hack Pokemon into your game.
Hope this cleared things up