The Enemy Within downloadable content pack for XCOM: Enemy Unknown will be adding new options for the players like nano-tech psionics and powered armor, but the enemy aliens will be getting an upgrade as well. One of the them is the Mectoid, which is a Sectoid in its own powered suit.
Naturally, the Mectoids are the aliens' answer to the human mech units, and with the new updated AI, the aliens will start going after higher priority targets, such as the obvious threats of humans lumbering around in giant robot suits.
XCOM: Enemy Within will be available on the 12th November 2013 for $29.99 USD. The DLC Commander Edition includes all XCOM: Enemy Unknown content plus the Elite Soldier and Slingshot add-ons, for $39.99 USD.