Cave Spider

The Blue Spider is a hostile mob that is believed to be introduced in Minecraft Beta 1.8. It is a spider with a blue retextured skin and is presumably more dangerous than its brown brethren. Blue spiders are much more rare than regular spiders. When bitten by a blue spider, your health bar turns yellow and gradually decreases. Blue spiders can only be found in Abandoned Mine Shafts, which will also be introduced in Beta 1.8. Blue Spiders are also smaller than regular spiders ; presumably so that they can fit through the mine shafts that they spawn in.

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.
 * In the Beta 1.8 preview, a Blue Spider was called a "poison spider", but "venomous spider" would be a more defining name.
 * The Blue Spider appears to be based off a Gooty Sapphire Ornamental Tree Spider or a Cobalt Blue Tarantula. Both are blue tarantulas; the former being called a Tree Spider (as Jeb said their name was originally) and both are known for their potent venom (as the Blue Spider in Minecraft is).
 * A possible cure for the spider venom could be milk, as tweeted by Jeb when someone suggested it.