Once you make it to Amberette Town in Pokemon X and Y you will go to the Fossil lab to find out more about Mega evolutions. From there you will be sent to the Glittering Cave to find the research assistant and he will give you the option between two fossils. You can only pick one and each one turns into a different Pokemon so read below to figure out which one you want!
Jaw FossilPokemon X & Y Jaw Fossil or Sail Fossil
If you pick the Jaw Fossil you will get Tyrunt who is a dragon/rock Pokemon. He can evolve into Tyrantrum and has the ability strong jaw which will make biting moves stronger(like Bite). He will start at level 20 and looks like a baby tyrannosaurus rex at the start and evolves into a bigger tyrannosaurus rex.
If you pick the Sail Fossil you will get Amaura who is a rock/ice Pokemon. He can evolve into Aurorus and has the ability refrigerate which makes any normal move into a ice move. He also starts at level 20 and looks like a brachiosaurus with a fin coming of his head.
Remember you can only pick one per save so be sure to take the one you want. Check back soon for more Pokemon guides!