Carracosta is a new water/rock pokemon in Pokemon Black and White. Carracosta has fairly good base stats, especially defense.
Carracosta can not be encountered in the wild. The only way to receive Carracosta is to choose the correct fossil in Relic Castle to receive Tirtouga. If you choose the Cover Fossil, you can bring it to Nacrene Museum and have someone there revive it into Tirtouga for you. Tirtouga will evolve into Carracosta at level 37.