Not really an easter egg but heres some places where u can find them-first4 appear on pokemon radar
ditto-route 218-
togepi-route 230
venonat-route 229
aron-fuego ironworks
giratina-turnback cave
spiritomb-get odd keystone,take it to that rocky hole thingy on route 209 (below solaceon).then talk to 32 people underground (you could also exit and enter 32 times and talk to the same person whick is a lot faster)
go back to the hole and press a. spiritomb will be there. its at a low level.
there is also a rare pokemon at the top of the volcano but i cant remember the name...
if theres any others that i havent included then comment and i will try and tell you