Legendary Pokémon other than Manaphy and Phione can't breed, so their eggs are rarely seen if at all. Eggs will always hatch in the earliest evolution stage, which can be a Baby Pokémon or a Basic Pokémon. In some cases, a baby Pokémon exists in the evolution line but an evolved form of that Pokémon will instead hatch if the correct incense is not used when breeding. While the artwork and anime show eggs have distinct characteristics depending on the Pokémon inside, the games present all eggs with the same sprite, except for Manaphy's.
Eggs used to hatch in level 5, but, with the introduction of Pokémon Diamond and Pearl, Pokémon began to hatch at level 1, in a way it would make sense for wild Pokémon to exist below level 5.
Pokémon Snap was the first game to introduce Pokémon eggs. The eggs of the legendary bird trio Articuno, Zapdos and Moltres could be found and hatched in the game, meaning legendaries have eggs, too, so there must be a way of getting them to breed.
This was the first generation to introduce the concept of breeding. An egg can be obtained from the Day Care Couple if the player leaves two Pokémon of the same egg group and opposite genders. The amount of time to hatch an egg depended on the number of steps the player had to make which varied with each species. All eggs in Gold and Silver are yellow with red spots. Aside from the eggs a player could create, a Togepi egg was given to the player early in the game, too.
The system here is the same as Gold and Silver's. The Day Care in Pokémon Ruby and Sapphire was found between Vendanturf Town and Mauville City while in Pokémon FireRed and LeafGreen there was one between Cerulean City and Saffron City incapable of creating eggs, and one for that purpose in Four Island. A Wynaut egg can be received at Lavaridge Town, and a Togepi egg in Five Island. Eggs are now cream-colored with light green spots.
The system and the look of eggs isn't really changed, but there is a new type of egg, Manaphy's, which can be received through Pokémon Ranger. A Happiny egg is received by a hiker on Hearthome City and a Riolu egg is given by Riley on Iron Island.