Genesect was revived from a Fossil and technologically enhanced by Team Plasma to create the most powerful Pokémon ever. N had the project cancelled and the P2 Laboratory closed because he felt that Pokémon, who he thought to be perfect beings, would lose their beauty if modified by science. However, Scientist Dudley kept working on it, angry that N denied his talent. This scientist reconstructed Genesect and installed the cannon on its back.
Stated there Genesect is originally a fossil Pokemon enhanced by science. No official word of what Pokemon it is but most(including I) believe it is Kabutops