In Pokémon Platinum, as with most of the Pokémon games, there are several Pokémon that you receive as Gifts from various computer characters within the game. Some are in eggs, others are the Pokémon straight up

#175 Togepi

The first of these Pokémon that you'll probably encounter is given to you by Cynthia. After you deal with the Galactic Grunts within Eterna City, Cynthia will reward you by giving you an egg of a Pokémon. Once hatched, it hatches into Togepi. Togepi is a Pokémon in the Sinnoh Dex, but you cannot find it in the wild until you have the National Dex

#133 Eevee

Next, when you get to Hearthome, if you go to the house next to the Pokémon Center, you will be introduced to Bebe. Bebe thanks you for using her system and gives you an Eevee. Eevees are not found in the wild in Pokémon Platinum until you have the National Dex so you'll have to breed it to get all the evolutions of it. before then
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#137 Porygon

In one of the northmost houses of Veilstone City, you will be given a Porygon by one of the people inside. Like Eevee, Porygon is not available in the wild in Platinum so if you wish to have more, you will have to breed.
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#447 Riolu

Another Pokémon that you cannot find in the wild, once you have helped Riley through Iron Island and defeated the Galactic Grunts, he will give you an egg of the Pokémon Riolu. Riolu is very rare and cannot be found in the wild on any game