To think about...
> All grass starters evolve into some sort of prehistoric creature

> Raptors are sort of like birds

> The official site leak suggests Rowlet becoming Grass/Ground and losing its Flying type...

> We don't have a Raptor Pokemon yet

Some arguments against it...

> Chespin doesn't become prehistoric

Yes it does. There are prehistoric creatures that look like Chesnaught.

> Order doesn't matter and the types belong to other Pokemon

They literally came out as soon as the starters will released. And until recently there where no Fighting or Ground Pokemon. Also those types where removed from the site for a time so it isn't a hoax... It wasn't meant to be there so they removed it. Also order does make sense. Why would Litten become Ground/Fire? Fire is its main type

> The dex shows Rowlets final evolution airborne

Flygon isn't a flying type...