Cave Spider

The Blue Spider is a hostile mob introduced in Minecraft Beta 1.8. It is a spider with a blue re-textured skin and is presumably stronger than its brown brethren. When bitten by a blue spider, your health gradually decreases. Blue spiders can only be found in abandoned mine shafts, which will also be introduced in 1.8. These shafts will have a unique spawning mechanism where a mess of cobwebs at the end of a corridor will protect a Blue Spider mob spawner. This generation will be rare and limited to spawning near Mine Shafts.

Trivia

 * It was originally going to be called a tree spider according to Jeb, but that wouldn't make any sense due to it spawning in mine shafts.