Mystery Eggs

If there's a Pokémon that cannot be hatched, it's possible that it's a Recruitable Pokemon instead.

Pokémon Eggs can be found in certain dungeons. These eggs hatch into unevolved forms of Pokémon found in that particular dungeon. When hatched, these Pokémon know four random Egg Moves, unless the "Parent" Pokémon knows an egg move, in which case it will know that. Eggs must meet the following requirements in order to hatch; Alternatively, eggs can be hatched at Joy, a NPC in all regions -- sans Tanren Undercity, for a fee. To use her, you must have 5,000 Poke per egg in your bag, but it only costs 4,000 Poke per egg.
 * Eggs must be hatched outside of a dungeon.
 * The player must have an open slot in his or her party.
 * The player must take a certain amount of steps.
 * The steps required vary between species, and may also vary depending on the dungeon where the egg was found.

List of Egg Pokémon
Below is a list of Pokémon that can be hatched from Eggs.

Trivia

 * The Easter Event was an event centered around hatching Pokémon eggs, and therefore has a couple aspects uinque only to it:
 * At one point it was possible to hatch a level 1 Mightyena.
 * At one point it was possible to hatch escort Pokémon, resulting in the recruitment of Pokémon that are otherwise unobtainable with the Partner Egg System.
 * It is possible to hatch a level 1 Shedinja with the Partner Egg System.