How do we know the names of egg groups?

Question

Or were they made up and now everybody uses the names of the egg groups?

Answer 1

If you want, here are the egg groups. But I assume you mean how do they determine them since they don't tell you them in the game. It goes by what nintendo has put up. In such games as pokemon stadium 2, they give a list of all the egg groups, so they have used that since.

http://pokemondb.net/mechanics/breeding