Aerodactyl is in the flying egg group, so you can breed it with a Pidgey.
And that is something I REALLY don't want to think about.
You can breed any pokemon that isn't in the "Undiscovered" egg group, which is typically only legendaries, baby pokemon, and a few odd pokemon, like Unown. Nidorina and Nidoqueen are in this eggroup, for unexplained reasons.
Only breedable legendary is Manaphy.
Ditto also can't breed with itself - catching only.
Otherwise, you can breed anything in the game.
Also, in the GTS, you only see pokemon you've seen in the game, but if you scroll down the list of letters to the bottom, there's an option to type the name of the pokemon you're looking for, and can trade for pokemon you haven't met.
BLESS YOUR ****ING HEART.