This is a somewhat controversial theory, some say it is Kabutops, some say  Scyther.  I personally see it much more likely to be Kabutops, and here is why.

First off is some circumstantial evidence, their body similarities.  Here is some fan art (artist unknown) with a gun-less Genesect next to Kabutops.

Notice the similarities in head shape, arms, and legs.  The claws on Genesect's arms look like Kabutops', with the lower bit of claw covered in steel.  The claws on the feet look like Kabutops' with the toe claw's shoved together and covered in steel.  They both have a similar abdomen, the part where it sticks out behind their body.  So, Genesect or Iron Kabutops?  Every difference can be explained by Team Plasma's meddling.  They are also about the same height without the cannon.

But Kabutops is a Kanto Pokemon!  Team Plasma would not have found its fossils in Unova.  I hear you, but you are wrong to think that.  In Black 2/White 2 inside Twist Mountain is a worker (miner?) that gives you one random fossil a day, including the Dome Fossil.  The Dome Fossil becomes Kabuto, Kabutops' pre-evolution.

So, what about Genesect's Bug typing?  I must say that it is tricky, but I believe Kabutops looks like Bug type material.  Plus, Team Plasma was trying to make a very powerful Pokemon, and could have altered Kabutops is some way to make him Bug/Steel type.  This is one of the best typings in the game, giving great coverage and only one weakness.  I think that covers that, and now to give you the best proof ever that Genesect is Kabutops.

Here is Genesect's Pokedex entry from Pokemon Black,  "Over 300 million years ago, it was feared as the strongest of hunters."  That is only the first half, but it tells us how old it is, and in one of the Pokemon canons N shows or states that its original form was revived from a fossil.  That eliminates Scyther as an option.  Many of Kabutops' Pokedex entries state that it was a good hunter.

Probability: 67%

That is all I have for this theory, let me know what you think.