Egg

Eggs are items that contain Pokémon. Eggs can be obtained in two ways: Buying them from a shop or breeding a level 100 Pokémon. Getting the egg through the latter method will boost the bred Pokémon's attack by 25% of its base attack.

Shop Egg Pool
 Kanto 

 Johto 

 Hoenn 

 Sinnoh 

Helpful Info
When you enter the next are the Pokemon rates change. Assuming you've just entered Sinnoh the Water Egg table changes from

In Hoenn
(Squirtle, Lapras, Staryu, Psyduck)

(Totodile, Wooper, Marill, Qwilfish)

(Mudkip, Feebas, Clamperl)

(Mudkip, Feebas, Clamperl)

In Sinnoh
(Squirtle, Lapras, Staryu, Psyduck)

(Totodile, Wooper, Marill, Qwilfish)

(Mudkip, Feebas, Clamperl)

(Mudkip, Feebas, Clamperl)

(Piplup, Finneon)

(Piplup, Finneon)

(Piplup, Finneon)

(Piplup, Finneon)

Which makes the chance of getting a Pokémon from the new region you entered 50% and dropping the total combination of all previous areas percent's down to 50%