Which Pokemon do I need to trade between X & Y in order to Catch 'Em All?

The reason there are two different versions of Pokemon, is that Nintendo generally want people to trade between the games in order to collect all the Pokemon of that region. (That and a helpful serving of money from the people who buy both games).

My question is, which Pokemon should I be catching two of, in order to trade them with my friends? Or to phrase it differently, which Pokemon are exclusive to a single version of the game?

Solution 1:

Barring Pokemon that cannot be found in either X or Y (of which I don't know yet), the following Pokemon must be traded between the two versions.

Exclusive to X

• Staryu
• Starmie
• Pinsir
• Houndour
• Houndoom
• Poochyena
• Mightyena
• Aron
• Lairon
• Aggron
• Sawk
• Swirlix
• Slurpuff
• Clauncher
• Clawitzer
• Xerneas

Exclusive to Y

• Shellder
• Cloyster
• Heracross
• Larvitar
• Pupitar
• Tyranitar
• Electrike
• Manectric
• Purrloin
• Liepard
• Throh
• Spritzee
• Aromatisse
• Skrelp
• Dragalge
• Yveltal

Notes

There are also a few Mega Evolutions that specific to each version, but those can be acquired by trading the item required (Mewtwonite X/Y, for example).