The Floating Rock Dragon is a bred dragon- this means you can't find it naturally and to get its egg you need to breed the earth and air dragons check Breeding dragons for more info. This is one of the more complicated breeds.
You can ride this draogn as any other (You need to use a Reinforced Saddle) click F to use the primary attack Burst and shoot out a piece of rock that deals 20-50 dmg click G, however to use the secondary attack sheild of rock which will down a 12 high 3 thick wall of stone infront of you.
This dragon must be fed 3 Gravel per 4 in-game days. If this isn't done in time, the dragon will become feral and hostile. In multiplayer, there is an exeption for this (See Feeding Dragons).