the one at the very very top? before bulbasaur? on its on with nothing either side of it?
that is the standard poke egg, where as the rest of these eggs are designed by the posted except the ones from the anime (eevee, togepi) in the games when ever a pokemon hatches from an egg, that egg appears despite what the species is, it gives a sense of surprise of what poekmon will come out if you forgot about the egg or got given it...however manaphy's egg is the only one that is different (it looks the same as the one depicted here, so I asumme thats where the poster got it from).
in otherwords, either it comes from any pokemon, or it will hatch into...missingno