Pokemon Debuts:
Kecleon appearedduring the Season 4(Jhoto League Champions)EP 205 "The Kecleon Caper".
To mkae things easier, here is Pokemon who appeared before their generation:
Generation II Pokemon:
Togepi - Season 1 (Origenal Series) EP51 "Who Gets To Keep Togepi?"
Marill - Pikachu's Vacation (Pokemon theFirst Movie: Mewtwo Strikes Back)
Snubbull - Pikachu's Vacation (Pokemon the first Movie: Mewtwo Strikes Back)
Donphan - Pokemon the First Movie: Mewtwo Strikes Back
Elekid - Pikachu's Rescue Adventure (Pokemon the Movie2000: The Power of One)
Lugia - Pokemon the Movie 2000: The Power of One
Slowking - Pokemon the Movie 2000: The Power of One
Generation III Pokemon:
Beautifly - Pokemon 3: Spell of the Unown*
Wailmer - Pikachu's PikaBoo (Pokemon 4Ever)
Volbeat - Camp Pikachu (Pokemon Heroes)
Duskull - Camp Pikachu (Pokemon Heroes)
Latias - Pokemon Heroes
Latios - Pokemon Heroes
Kecleon - Look Above.
Generation VI Pokemon:
Wynaut - Camp Pikachu (Pokemon Heroes)
Munchlax - Destiny Deoxys
Lucario - Lucario and the Mystery of Mew
Bonsly - Lucario and the Mystery of Mew
Mime Jr. - Lucario and the Mystery of Mew
Weavile - Lucario and the Mystery of Mew
Chatot - Pokemon Ranger and the Temple of the Sea
Buizel - Pokemon Ranger and the Temple of the Sea
Mantyke - Pokemon Ranger and the Temple of the Sea
Manaphy - Pokemon Ranger and the Temple of the Sea
* It was briefly seen in the book Molly and her father were looking through.
BTW, if you want to add anything I missed, then go right ahead.
Arceus: The Creation/Alpha Pokemon:
Okay, yes Arceus is the strongest Pokemon. However it is NOT a god Pokemon. It is known as the Creation/Alpha Pokemon and is supposedly the first Pokemon to exist. Yes it did create Dialga, Palkia, Mesprit, Uxie, and Azelf which created the Universe and the mental properties (Emotion, Will, Knowledge). However may I remind you that this is a Sinnoh Myth. No one really knows where these 6 pokemon came from and did these things. For all we know Arceus could just behave a bigego, Palkia likes to fly around in space, Dialga likes time travelling, Mesprit could enjoy robbing you from emotion, Uxie could enjoy robbing you of knowledge, and Azelf could enjoy robbing you from moving. Or I could be taking the word myth as what it means and not what the show wants it to be again like I did in the "What Is A Pokemon Master" thread.