Water Skipper

The Water Skipper (more commonly known as a Water Strider) is a bug that has the ability to walk on water.  It would be fun if these guys would spawn on the water in Rivers and Swamps.  They are only hostile towards arthropod based mobs, such as Spiders, Cave Spiders, and Silverfish if those mobs come close to the water or go in the water.  The player wouldn't be able to tame them, but they could be bred with spider eyes.  They wouldn't despawn like some mobs (bats, squids, etc).