to get a Riolu egg in Pokemon platinum you have to go to iron island by taking the ferry from Canvalve city then find riley in the cave and battle with her. then at the end she will give you the egg but you must have room in your party.
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you get a riolu which can evolve into lucario
Riley will give you and egg on Iron Island
from Riley he gives you a Riolu egg and you can evolve it at high friendship
around 10000 steps
go to iron island and find riley. go battle the team galaactic guys and after he will give you a riolu egg then just hatch it to get riolu