This Pokémon was the first created.
Rhydon?
Actually, I think it's Mew. Nintendo applied for a trademark on "Mew" in 1990, a full nine years before Pokemon became a global phenomenon.
Actually, Mew was the first to have its name applied for a trademark but according to the Official Nintendo Magazine Australia and New Zealand (Issue 23, November 2010, page 057):
What was the first Pokémon ever created? Pikachu? Nope. Charmander? Afraid not, good sir. According to Ken Sugimori, the man behind Pokémon artwork, it was actually Rhydon, That's why the generic icon for Pokémon in the game looks like a Rhydon, and why all the statues in the Kanto gyms look like a Rhydon. Incedentally, in case you were wondering, the last one created in the original game was Victreebel.
This one is the first (and as far as I know, the only) Pokémon with a Canonical Scientific Name.
Oddish. (Oddium Wanderus)
Kabutops has one too (Kabutops maximis), and it appeared an anime episode before FireRed and LeafGreen were released, so it was the first.